Anton von Leeuwenhoek and his Perception of Spermatozoa

Adapted from an article by E. G. Ruestow, J. History of Biology 16: 185-224.

Leeuwenhoek may have been the model for his friend, Vermeer, in Vermeer's picture, The Geographer.

In the seventeenth and eighteenth centuries, no scientific question was more laden with religious and philosophical overtones than the problem of generation. This was the question that asked if and how a mechanistic nature, devoid of spirit, could engender the purposeful complexity of living organisms, including man.

It is not surprising, then, that the learned (and probably the unlearned) public expected much from the new microscopists—men such as Marcello Malpighi, Jan Swammerdam, and Antoni van Leeuwenhoek. Leeuwenhoek's conclusions, made from observations with microscopes whose resolution was not bettered until the nineteenth century, unsettled a recently recast consensus. Moreover, it suggested that the microscope may not be the tool that would solve these longstanding problems.

Leeuwenhoek's knowledge of the religious and philosophical debates among the learned communities was scanty at first, but expanded dramatically during the half-century of his microscopic research. A tradesman in Delft, he lacked any university education and knew no language other than Dutch. In 1673, however, Leeuwenhoek's fellow townsman, the prominent young anatomist Regnier de Graaf, brought him to the attention of the Royal Society in London as a maker of exceptional microscopes (1).

It was a turning point in Leeuwenhoek's life and the beginning of a long and loyal relationship with that eminent scientist. His initial interest in the microscope probably arose from his having fun employing his manual skills and technical ingenuity towards building better optics, but eventually, as he acquired new and learned friends, he developed a taste for speculation as well. He became aware that his thinking was not that of the "common man" and would assert in later years that his writings were only for those of a "philosophical" mind (2).

Although initially sensitive to his being a tradesman, Leeuwenhoek had a self-assured and assertive streak as well. He responded to early skepticism in intellectual circles abroad with unruffled self-confidence. On one issue, whether the male semen or the female egg was the origin of mammalian generation, Leeuwenhoek soon took a position of aggressive opposition to the general consensus of the medical community, an opposition that at times bordered on defiance of the whole academic tradition of learning. Convinced by his microscopic observations of the primary role of the semen, Leeuwenhoek wrote that his work superseded both that of William Harvey and even his friend de Graaf (3). He put greater faith in his own microscopes and intelligence than in the speculations of all philosophers before him.

Among the substances Henry Oldenburg had urged his new correspondent to study with his microscopes was semen. Leeuwenhoek initially did not want to do this. He questioned the propriety of writing about semen and intercourse. When he first trained his microscope on semen, Leeuwenhoek discovered what he then took to be globules. However, he so disliked the prospect of having to discuss it that he quickly turned to other matters (4). Three or four years later, however, in 1677, a student from the medical school at Leiden, Johan Ham, brought him a specimen ostensibly of semen in which Ham himself had found small animals with tails, which Leeuwenhoek now observed as well (5). Leeuwenhoek resumed his own observations and in his own semen—acquired, he stressed, not by sinfully defiling himself but as a natural consequence of conjugal coitus—observed a multitude of "animalcules," less than a millionth the size of a coarse grain of sand and with thin, undulating transparent tails (6).

A month after Ham's visit, Leeuwenhoek described these observations in a brief letter to Lord Brouncker, president of the Royal Society. Still uneasy about the subject matter, he begged Brouncker not to publish it if he thought it would give offense (7). Although the elder Huygens wrote Leeuwenhoek that the observations in the letter to Brouncker were as important as anything that had yet been seen, Leeuwenhoek himself did not at first attach exceptional significance to the spermatozoa. He appeared to see them as another example the microscopic life he was discovering in various waters and infusions. It is possible that Christiaan Huygens and Ham may well suspected the relevance of sperm to reproduction before Leeuwenhoek did (8). Nonetheless, Leeuwenhoek's letter to Brouncker challenged the prevailing ideas about animal generation, supported indeed by the authority of William Harvey and Leeuwenhoek's late friend, de Graaf.

Ironically, Leeuwenhoek's challenge to this authority was based initially on images that still remain unexplained (9). Besides the spermatozoa, Leeuwenhoek claimed to see in the thicker part of the semen a large variety of microscopic vessels, and he was persuaded that they were nerves, arteries, and veins. "Indeed, I have seen so many of these vessels that I believe I have observed more in a single drop of semen than an anatomist encounters during a whole day of dissection." (10). His early observations of semen stressed the importance of these vessels. Although they have not been reported by any other observer, Leeuwenhoek's drawings of these vessels show them to be clearly delineated (11).

These observations convinced Leeuwenhoek that the human body contained no vessels that were not already in the semen, and he seems to have posited a very delicate, preembryonic vascular system that included the beginnings of the vital organs and already carried the animal spirits (12). Interestingly, Preformationism had been on the rise during the seventeenth century, but most of the Preformationists thought the egg was the seed which gave rise to animal. Only subsequently, perhaps, did Leeuwenhoek realize how unorthodox it had become to locate that preformation within the semen rather than in the egg. In any event, he waged a persistent campaign to prove that he was right and recent opinion wrong. In this process, he eventually abandoned the microscopic vessels for what was an even more unorthodox commitment to the spermatozoa.

It was Nehemiah Grew who raised doubts about the vessels and informed Leeuwenhoek about popular scientific opinion. Grew wrote Leeuwenhoek that a new synthesis had emerged postulating that the semen was "only the vehicle of a certain highly ethereal [animalis] and volatile spirit, which makes vital contact with the site of conception, that is, the female egg (13)." This view had melded the views of the Aristotelians (who assumed that menstrual blood in the uterus formed the fetus under the influence of the semen), and the Hippocratics (who posited a female semen as well, that mixed with the male semen in the uterus). In the Netherlands, philosophical thinkers usually adhered to the latter view, often presuming that the female semen derived from the female "testicles" (in fact the ovaries) and reached the uterus through the fallopian tubes (14).

The question was opened again by Harvey's Exercitationes de generatione animalium (15). Harvey dissected numerous animals and had seen nothing—no semen of any variety nor any egg— in the uterus immediately after mating (16). In deer, it took weeks for anything visible appear, a membranous sack extending throughout the uterus and filled with fluid, which Harvey thought was an egg (17). This duration between mating and the first observable uterine entity was subsequently confirmed in the Netherlands (in rabbits) by de Graaf and Anthony Everaerts, but it strained the ingenuity of the medical establishment (18). Shortly thereafter, a circle of anatomists associated with Leiden (and including Jan Swammerdam, the Dane Nicolaus Steno, and de Graaf), redefined the female testicles as ovaries and the ovarian follicles as eggs (19).

The idea that man and other mammals produced eggs was a startling novelty and not without its problems. Especially problematic was the question of how the "eggs" got from the ovary to the fallopian tube and then passed through its initial narrows to the uterus (20). It was de Graaf who recognized that the true egg was in fact much smaller than the whole follicle and discovered in rabbits what he took to be the egg in the middle of the fallopian tube (21). Soon after de Graaf's publication most scholars took the evidence for mammalian eggs as unassailable.

Thus, the new pieces fit together, but only in a manner wherein the male semen had merely an ethereal (and not a corporal) nature. Not only was semen absent in the uterus, but it was thought to impregnate the eggs while still in the ovary. Thus, the impregnating principle of the semen was described as spirit, vapor, odor, or "irradiation (22)." It is probable that Leeuwenhoek remained ignorant of the history, implications, and details of the discussions of animal generation in academic circles. Upon becoming a disputant in these controversies, Leeuwenhoek sought to end them by doing his own dissections. The response to his conception of the role of the semen assured his further education, however, and it also prompted him, characteristically, to take up the knife himself.

Soon after he had heard from Grew, Leeuwenhoek dissected genital organs of animals and campaigned de Graaf's hypothesis. Unaware of de Graaf's distinction between the follicle and the actual egg, Leeuwenhoek immediately proclaimed his astonishment that so much had been ascribed to vesicles that contained nothing but a watery fluid with globules that could be dislodged only with violence to surrounding tissues (23). [Influenced in part by the early engraving he had seen, most likely Swammerdam's widely exposed but uncharacteristically fanciful depiction of eggs hanging on vessels within the ovary, Leeuwenhoek had expected to find them similarly loose within a hollow organ (24).] He argued that they could not possibly pass through the narrow lumina of the fallopian tubes and could be found neither in these tubes nor in the uterus (25). He declared the mammalian ovaries to be useless ornaments and the sole function of the uterus and the female sex was to receive and nourish the masculine seed (26). He became committed to demonstrating that the notion of mammalian eggs was a foolish fantasy sustained by nothing better than obstinacy (27). However, those dissections also produced a substantial change in the alternative Leeuwenhoek proposed.

At Grew's request, Leeuwenhoek had proceeded to look for spermatozoa in other animals, and he found them in great numbers in the semen of rabbits, dogs, and fish (28). He did not yet associate the spermatozoa with impregnation, however, or even intrinsically with the semen itself, since he believed during his early observations that they arose, not with the microscopic vessels in the testicles, but separately within the "male member (29)."

For his early observations of semen, Leeuwenhoek relied on residue acquired after the act of mating, but in the spring of 1679 he removed the testicles during the dissection of a male hare and had cut the vas deferens (30). He found it filled with an unbelievable multitude of spermatozoa (motionless, however, since the animal had been dead for several days). Cutting the testicle itself, he also found the spermatozoa in the filaments of which it was composed (31). Similar dissections followed on dogs and fowl, which convinced him not only that the spermatozoa originated in the testicles but that the testicles, in fact, had no other purpose (32). The microscopic vessels, meanwhile, inexplicably faded away, while the structure and role of the sperm was emphasized (33).

His continued observations revealed enormous numbers of spermatozoa in the testicles and vasa deferentia of a wide variety of other animals (34), further confirming for Leeuwenhoek the essential identification of spermatozoa and semen. He was now ready to make pronouncements on humans. In 1683, he wrote (35), he was now more certain than ever "that man comes not from an egg but from an animalcule in the masculine seed." He soon was able to answer even Harvey. Harvey had not used a microscope. Leeuwenhoek reported in early 1685 that with a good microscope he had found large numbers spermatozoa in the uterus and fallopian tubes of recently mated dogs and rabbits (36).

He could not get evidence, however, that the sperm was indeed the precursor of the embryo. But if observations were unable to confirm the continuity of spermatozoon and embryo, metaphysics could bridge the gap. In 1685, Leeuwenhoek reported finding within the uterus of a rabbit mated six days before, a small, round vesicle the size of a millet grain that other authors, he said, would doubtless have called an egg but which he assumed to have derived from a spermatozoon (37). Opening the vesicle—evidently a blastocyst, which de Graaf had observed in the rabbit only three days after mating and had indeed considered the egg—he briefly supposed he might have seen the figure of a rabbit a thousand times smaller than a grain of sand but abandoned this thought when he failed to find any such suggestion in other similar vesicles (38). He concluded, nonetheless, that a body "animated [besielt] with a living soul from the male seed" had been enclosed within. Leeuwenhoek identified the "living soul" of an animal with an endowed capacity for movement and he repeatedly insisted, therefore, on the soul in each spermatozoon (39). This being so, he wrote, it was a thousand times more probable that the living soul in the spermatozoon remained there as the body of the spermatozoon changed, so that the parts of the egg passed to the spermatozoon, rather than the living soul being transferred to the parts of the egg (40). Leeuwenhoek had identified the impregnating (and ensouling) principle of semen with microscopic animals.

The astonishing activity of the spermatozoa testified that they not only had souls, but that they had complex anatomic structures. Indeed, wrote Leeuwenhoek in 1679, there may be as many parts in the sperm as in the human body itself (41). Perhaps implicit before but ambiguously expressed (42), Leeuwenhoek's presumption of the preformed man or animal within the spermatozoon was explicitly articulated in 1685, the same year as his commitment to the spermatozoon's living and persisting soul (43).

If spermatozoa were really seeds that were nourished by the egg and uterus, then he might be able to make the structural comparison between plant seeds and their animal (spermatic) counterpart. Like most of his contemporaries, Leeuwenhoek failed to recognize that plant seeds were themselves the product of sexual union (44), and, arguing that the propagation of varied living things was essentially the same, he insisted on an analogy between the embryo plant in the seed and the spermatozoon (45). He declared in 1685 that his observations, showing the embryos inside plant seeds, had rendered the propagation of animals from spermatozoa incontrovertible (46). Indeed, he claimed (47) that the vessels in the seeds of trees were the precursors of the branches of the adult tree. How else could the seed produce the tree? Within each seed, consequently, was hidden all the perfection of the entire tree, including its fruit and seeds (48). These observations within the seed continued throughout the years to help sustain the conviction that "all the beginnings of the perfection with which a large creature (that has come from such a small animal) is provided," indeed the "entire man," lay already within the spermatozoon (49).

This intuitive conviction in headier moments inspired vivid expectations. "I have in fact imagined," he also confessed in 1685 (50), that I could say as I beheld the animalcules in the semen of an animal that there lies the head and there as well the shoulders and there the hips; but since these notions have not the slightest shred of certainty, I will not yet put forward such a claim, but hope that we may have the good fortune to find an animal whose male seed will be so large that we will recognize within it the figure of the creature from which it came."

The seeming simplicity of his intuition, however, was entangled by difficulties that blur its ultimate and precise meaning for Leeuwenhoek. When he confirmed at the turn of the century that his contention was, indeed, that a lamb lay enclosed in the spermatozoon of a ram, he affirmed that this was true even though the parts within the spermatozoon did not resemble a lamb, whose form they assumed only when nourished in the womb (51).

When in 1699 a celebrated hoax reported and illustrated the discovery of the lineaments of a man within a spermatozoon, Leeuwenhoek wrote to the Royal Society in order to deny it (52). If a spermatozoon were provided with all the perfected members capable of being recognized through the microscope, he reasoned, would they not remain in this same state of perfection as they grew? But in the developing chick, he pointed out (testifying as well to his broadened education), Malpighi had shown that this did not occur. Leeuwenhoek's own observations of early embryonic development had offered no less striking evidence against the animal's presence in the spermatozoon as a miniature replica; indeed, in extreme instances, those observations would have been perceived by observers with other predispositions as testifying conclusively that there was no persisting animal structure there at all.

Most unexpected, however, were observations Leeuwenhoek reported in 1688 of the embryonic development of the frog. Without any cell theory in which to place his results, Leeuwenhoek described the eggs as consisting initially of an inconceivable number of rather complex globules in a watery fluid (53). Even the dissected body of a young free-swimming tadpole seemed to be composed of globules alone, with no intestines, nerves, or vessels. In later years, however, he insisted that although the form of the frog was not discovered in the tadpole, the frog lay enclosed there nonetheless (54), which must have meant to Leeuwenhoek the persisting, if altered and invisible, structure of the former spermatozoon, as well. Observations could apparently support but not refute so fundamental a preconception.

The search for the ultimate confirmation of this preconception inspired recurrent assaults on the limits of his microscopes and ingenuity. The discovery of the future animal in one form or another within the spermatozoon would have been, after all, the crowning achievement of Leeuwenhoek's microscopy; and though he failed, it was not for lack of trying. He attempted to strip the spermatozoa of their outer skin with a small moistened brush as they lay dried upon the glass (55). The effort failed, and repeated attempts to probe their inner structure yielded little more than suggestions of globular shapes.

Leeuwenhoek at times suspected that he might fleetingly have glimpsed something more (56), but by the end of the century he accepted the barriers he confronted as final. A "great secret" was hid within both the seeds of plants and the semen of animals, and by the mid-l690s, his efforts to probe ever deeper with his microscopes had only convinced him that this hidden secret could not be reached (57). It was inconceivable, he wrote in 1699, "that human ingenuity will penetrate so deeply into that great secret that, by chance or by the dissection of the animalcule in the semen, we will come to see the entire man." Yet he had no doubt that the entire man—in whatever form—was there (58).

Literature Cited, Including Footnotes

1. Regnier de Graaf to Henry Oldenburg, 18 April 1673, in Oldenburg, Correspondence, ed. and trans. A. Rupert Hall and Marie Boas Hall (Madison: University of Wisconsin Press, 1965), IX, 602-603.

2. Leeuwenhoek to Antonie Heinsius, 17 December 1712, in Leeuwenhoek, Send-Brieven (Delft: Adriaan Beman, 1718), p. 22; Leeuwenhoek to the Royal Society, 10 July 1686, A.d.B., Vl, 128-129; Leeuwenhoek to Hendrik van Bleyswyk, 28 December 1695, in Leeuwenhoek, Sesde Vervolg der Briewen (Delft: Henrik van Krooneveld, 1697), p. 186; Leeuwenhoek to the Royal Society, 15 June 1717, Send-Brieven, p. 372; Leeuwenhoek to the Royal Society, 20 November 1717, ibid., p. 453.

3. Leeuwenhoek to Nehemiah Grew, 18 March 1678, ibid., II, 328-339, esp. 332-339.

4. Leeuwenhoek to William Brouncker, November 1677, ibid., II, 290-291.

5. Ibid., pp. 280-283; Leeuwenhoek to Lambert van Velthuysen, 13 June 1679, Ibid., III, 74-75; Leeuwenhoek to Herman van Zoelen, 17 December 1698, in Leeuwenhoek, Sevende Vervolg, p. 65.

The donor of the specimen was a patient afflicted with gonorrhea, and since the discharge symptomatic of the disease was often identified with semen, it is not unlikely that the specimen was in fact this gonococcal discharge. (Regarding the instruction at Leiden in particular, see Ham's own relative Theodorus Craanen, Tractatus physico-medicus de homine [Leiden: Petrus vander Aa, 1689], p. 750, and Gysbertus van Tol, Disputatio medica inauguralis, de gonorrhoea virulenta [Leiden: Vidua & Haeredes Johannis Elsevirii, 1674], Caput I, S. 5; see also Regnier de Graaf, Tractatus de virorum organis generationi inservientibus, in Opera omnia [Leiden: Officina Hackiana, 1677l, pp. 88-89.) I am grateful to Dr. F. Marc Laforce of the university of Colorado School of Medicine for reassurance that it is not unusual to find spermatozoa in such discharge.

It is possible that Ham himself had already made other observations of spermatozoa in healthy specimens of semen, for Fridericus Schrader of Helmstad, who had enrolled in the medical school at Leiden shortly after Ham's visit (his second, in fact) to Leeuwenhoek and graduated two years later, wrote in 1681 that his "very dear friend" Ham had first discovered spermatozoa in the Netherlands in the semen of a rooster. (Album studiosorum Academiae Lugduno Batavae MDLXXV-MDCCCLXXV [The Hague: Martinus Nijhoff, 1875], col. 616;"Catalogus promotorum," in P. C. Molhuysen, ed., Bronnen tot de Geschiedenis der Leidsche Universiteit [The Hague: Martinus Nijhoff, 1913-19241,111, 331.) Ham had also told him, Schrader added, that he had found all the spermatozoa dead in the semen of those suffering from "virulent" gonorrhea. (Fridericus Schrader, Dissertatio epistolica de microscopiorum usu in naturali scientia & anatome [Gottingen: Sumptibus Bartholdi Fuhrmanns, typis Johannis Christophori Hampii,1681], pp. 34-35.) On 12 January 1679 in Bologna, moreover, Marcello Malpighi noted the report of a "German friend" (Schrader?) that "animals like extremely small toads, dead and deprived of motion," could be observed in the semen of those with gonorrhea. (Howard B. Adelmann, Marcello Malpighi and the Evolution of Embryology [Ithaca, N.Y.: Cornell University Press, 1966] ,I, 420.) According to Leeuwenhoek, however, both he and Ham had observed living spermatozoa in the specimen that the latter had brought, and Ham had judged that they lived perhaps twenty-four hours; Ham also reported having observed them dead after the patient had taken turpentine (Leeuwenhoek to William Brouncker, November 1677, in Leeuwenhoek, A.d.B. ,II, 282-283.) See also note 24.

6. Leeuwenhoek to William Brouncker, November 1677, in Leeuwenhoek,A.d.B., II, 284-291.

7. Leeuwenhoek to William Brouncker, November 1677, in Leeuwenhoek, A.d B., 11, 290-293. He was apparently reassured, for Leeuwenhoek's subsequent letters would deal unabashedly with his investigations of sex-related matters, and in 1679 he showed the spermatozoa of a dog to no one less than the visiting Duke of York. (Leeuwenhoek to Robert Hooke, 13 October 1679, ibid., lll, 106-109.) But he would continue on occasion to censor his speculations, for he feared not only their impropriety but "that the world, which is coarse and vicious enough, might use the knowledge of nature for its own ruin and increasingly debauch itself in depravity." (Leeuwenhoek to Pieter Rabus, 30 November 1694, in Leeuwenhoek, Vifde Vervolg, p. 13.) Such scruples were not peculiar to Leeuwenhoek or his social class (Text of letter from Constantijn Huygens to Leeuwenhoek, 8 December1677, given in Leeuwenhoek to Herman van Zoelen, 17 December 1698, in Leeuwenhoek, Sevende Vervolg, pp. 68-69).

8. Christiaan expressed his sense of the importance of the spermatozoa and their relevance to the generation of animals in announcing their discovery the following year in Paris. (Christiaan Huygens, "Extrait d'une Lettre de M. Huguens de l'Acad. R. des Sciences l'Auteur du Journal, touchant une nouvelle maniere de Microscope qu'il a apport de Hollande," J. Savans, 15 August 1678, vol. 6, Amsterdam ed. [1679], 347.) Fridericus Schrader (see note 17) related that Ham, having first discovered the spermatozoa in the semen of a rooster, told him that he had also investigated the semen of sterile men, finding in it no spermatozoa at all, as well as the semen of those suffering from virulent gonorrhea: (Fridericus Schrader, De microscopiorum usu, pp. 34-35.) If this is so, such a line of inquiry certainly suggests an interest in a possible connection between the spermatozoa and generation, although the date of these investigations is not disclosed.

9. Carlo Castellani, "Spermatozoan Biology from Leeuwenhoek to Spallanzani," J. Hist. Biol. 6 (1973), 40; Leeuwenhoek, A.d.B., II, 294n33, 295n31.

10. Leeuwenhoek to William Brouncker, November 1677, in Leeuwenhoek, A.d. B., II, 292-295.

11. Leeuwenhoek to Nehemiah Grew, 31 May 1678, ibid., pp. 364-367, and "Tables" 18 and 19.

12. Leeuwenhoek to William Brouncker, November 1677, ibid., pp. 292-297; Leeuwenhoek to Nehemiah Grew, 18 March 1678, ibid., pp. 336-337. The English translation given in A.d B., 11, for Leeuwenhoek's letter of 31 May 1678 to Nehemiah Grew introduces a very misleading idea. Leeuwenhoek's reference on p. 362 to spermatozoa among the ostensible vessels of the semen acquires in translation (p. 363) the sense that the spermatozoa came to be among the vessels "by their escaping from the vessels" when the latter were broken, suggesting that the spermatozoa had been contained within the vessels (see 362n29 and 363n9 in this volume of the A.d.B.). I find nothing in the original Dutch passage to justify the introduction of the cited phrase or the attendant implication. Leeuwenhoek initially wrote that he had observed the spermatozoa primarily in the thinner, fluid part of the semen around the thicker part, which, composed of vessels, was, in fact, too densely packed, he surmised, to allow the spermatozoa to move in it. (Leeuwenhoek to William Brouncker, November 1677, ibid., pp 284-287, 292-293.)

In his letter to Grew, Leeuwenhoek is attempting to explain how some spermatozoa were also found among the vessels. The answer he offers is that when the vessels were broken apart in the semen spilled as the male animal mounted the female, the spermatozoa were able to swim in among them. Since he had earlier observed (or so he reported) that these vessels, when exposed to the air for a few moments, turned into a watery substance and oily globules like those he had seen among the putative vessels of the spinal marrow as well, he conjectured that the vessels of the semen might, in fact, carry animal spirits (ibid., pp. 294-297), presumably because the nerves and supposed vessels of the spinal cord were widely believed to carry these spirits. Leeuwenhoek indicated that the "body"-that is, the head-of the spermatozoa he had drawn for the Royal Society was perhaps slightly thicker than the prominent vessels of the semen he had drawn, the smaller of which, however, were so small as to escape his sight. (Leeuwenhoek to Nehemiah Grew, 31 May 1678, ibid., pp. 364-367.) True to the near-equivalence of diameters thus suggested, he described the diameter of these prominent vessels as less than a hundredth the diameter of a large grain of sand (ibid.) and also judged that such a grain of sand would be larger in volume than a million spermatozoa, (Leeuwenhoek to William Brouncker, November 1677, ibid., pp. 286-287.)

13. [Nehemiah Grew], "Auctoris ad Observatorem Responsum," Phil. Trans., 142 (December 1678-February 1679), 1043. Regarding Grew's identity as the author of this response, see Leeuwenhoek to Nehemiah Grew, 18 March 1678, in Leeuwenhoek, A.d.B. II, 326-333.

14. Jacques Roger, Les Sciences de la vie dans la pensée française du XVIII siélce, 2nd ed. (Paris: Armand Colin, 1971), pp. 53-63.

15. William Harvey, Exercitationes de generatione animalium (London: Typis Du-Gardianis, impensis Octaviani Pulleyn, 1651), p. 137.

16. Ibid., pp. 226-229.

17. Ibid., pp. 229-231.

18. Anthony Everaerts, Novus et genuinus hominis brutique animalis exortus (Middelburg: Franciscus Kroock, 1661), pp. 31-32, 35, 51; Regnier deGraaf, De mulierum organis generationi inservientibus tractatus novus, in Opera omnia, p. 397. Also, Harvey, Exercitationes degeneratione animalium, p. 228.

19. Nicolaus Steno, "Historia dissecti piscis ex canum genere," in Elementorum myologiae specimen, seu musculi descriptio geometrica (Florence: ex typographia sub signo Stellae, 1667), pp. 117-118; Johannes van Horne, Suarum circa partes generationis in utroque sexu observationum prodromus (Leiden: Gaasbekios, 1668), p. 181; Jan Swammerdam, Miraculum naturae (Leiden: Severinus Matthaei, 1 672), pp. 1 9-20 , 45; de Graaf, De mulierum organis, pp. 298, 302-303. The Leiden circle was apparently anticipated by several years, however, by Willem Langly, physician at Dordrecht; see Justus Schraderus, Observationes et historiae omnes & singulae e Guiljelmi Harvei libello De generatione animalium excerptae (Amsterdam: Abrahamus Wolfgang, 1674), "Praefatio," pp. [xivl-[xv]. Also, Swammerdam, Miraculum naturae, pp. 19, 22-23; Ysbrand van Diemerbroeck (Anatome corporis humani, plurimis novis inventis instructa [Utrecht: Meinardus a Dreunen, 1672], p. 218), offered other contrary arguments.

21. De Graaf, De mulierum organis, pp. 348-349, 400-401.

22. [Theodorus Craanenl, "Exortus sive generatio hominis novus & genuinus," Oeconomia animalis ad circulationem sanguinis breviter delineata (Gouda: Guilhelmus vander Hoeve, 1685), p. 5; Diemerbroeck, Anatomes corporis humani in Opera omnia, pp. 132, 159, 163,168, 179,220; StevenBlanckaert, Medicinae institutiones, in Opera medica, theoretica, practica et chirurgica (Leiden: Cornelius Boutestein & Jordanus Lugtmans, 1701), 1, 219, 273; Wolferdus Senguerdius, Philosophia naturalis, 2nd ed (Leiden: Danieles a Gaesbeeck, 1685), p. 399; de Graaf, Tractatus de virorum organis, pp. 88, 92-93; idem, De mulierum organis, pp. 346-347; Swammerdam, Bybel der Natuure, 11, 514-517; Antony Nuck, Adenographia curiosa et uteri foeminei anatome nova (Leiden: Jordanus Luchtmans, 1691), p. 69.

23. Leeuwenhoek to Nehemiah Grew, 18 March 1678, ibid., ll, 342-347.

24. Ibid., pp. 342-343; Swammerdam, Miraculum naturae, facing p. 34; A. Schierbeek, Jan Swammerdam: Zijn Leven en zijn Werken (Lochem: "De Tijdstroom," 119461), p. 114 and plate 5, facing p. 48. See also Gerard Blaes (Blasius), Ontleeding des menschelyken Lichaems (Amsterdam: AbrahamWolfgangh. 1675), plate 17, fig. 1; Steven Blanckaert, De nieuw hervormde Anatomie 12nd ed. (Amsterdam: Jan ten Hoorn, 1686), p. 463.

25. Leeuwenhoek to Christopher Wren, 22 January 1683, in Leeuwenhoek, A.d. B., IV, 12-13; Leeuwenhoek to the Royal Society, 30 March 1685, ibid., V, 162-169, 188-191, 194-197. Leeuwenhoek was later aware that both de Graaf and Malpighi had identified the ovarian follicle as the source of the ovum but not the ovum itself (Leeuwenhoek to the Royal Society, 19 March 1694, in Leeuwenhoek, Vierde Vervolg, pp. 659-660) but he did not alter his argument. (Leeuwenhoek to Gottfried Wilhelm Leibniz, 19 May 1716, in Leeuwenhoek, Send-Brieven, pp. 210-211;Leeuwenhoek to Gottfried Wilhelm Leibniz, 17 November 1716, ibid., p. 301.) Regarding Malpighi, see Marcello Malpighi, "Praeclarissimo & eruditissimo Viro D. Jacobo Sponio . . . ," Phil. Trans., 161 (20 July 1684), 637-641; Adelmann, Marcello Malpighi, 11, 853, 859. With respect to de Graaf, see note 45.

26. Leeuwenhoek to the Royal Society, 19 March 1694, in Leeuwenhoek, Vierde Vervolg, pp. 663, 669-670; Leeuwenhoek to Gottfried Wilhelm Leibniz, I 9 May 1716, in Leeuwenhoek, Send-Brieven, p. 211; Leeuwenhoek to Gottfried Wilhelm Leibniz, 17 November 1716, ibid., p. 304. See also Leeuwenhoek to the Royal Society, 30 March 1685, in Leeuwenhoek, A.d.B., V, 204-205; Leeuwenhoek to the Royal Society, 13 July 1685, ibid., pp. 248-249; Leeuwenhoek tothe Royal Society, 10 June 1686, ibid., VI, 120-123. Leeuwenhoek did believe, however, that the nourishment provided by the female in the uterus could alter the animal considerably. (Leeuwenhoek to the Royal Society, 13 July 168S, Ibid., V, 250-251.)

27. Leeuwenhoek to the Royal Society, 30 March 1685, in Leeuwenhoek, A.d B., V, 158-159, 164-165, 170-171; Leeuwenhoek to the Royal Society, 10 June 1686, ibid., Vl, 122-123; Leeuwenhoek to the Royal Society, 19 March 1694, in Leeuwenhoek, Vierde Vervolg, p. 666.

28. [Grew], "Auctoris ad Observatorem Responsum," p. 1043; Leeuwenhoek to Nehemiah Grew, 18 March 1678, in Leeuwenhoek, A.d. B., II, 326-327, 340349; Leeuwenhoek to Nehemiah Grew, 21 February 1679, ibid., pp. 418-421.

29. Leeuwenhoek to Nehemiah Grew, 25 April 1679, in Leeuwenhoek, A.d. B. III, 18-19.

30. Leeuwenhoek to Nehemiah Grew, 25 April 1679, ibid., III, 8-9.

31. Ibid., pp. 8-11.

32. Ibid., pp. 18-21.

33. Ultimately, he completely repudiated his former accounts of the vessels in the semen (Leeuwenhoek to Christopher Wren, 22 January 1683, Ibid., IV, 8-11) concerning which he had once told the Royal Society he expected them to trust him. (Leeuwenhoek to Nehemiah Crew, 18 March 1678, ibid., II, 336-339.)

34. Leeuwenhoek to Nehemiah Grew, 25 April 1679, ibid., 111, 10-l9; Leeuwenhoek to Robert Hooke, 5 April 1680, ibid., pp. 202-209; Leeuwenhoek to Robert Hooke, 12 November 1680, ibid., pp. 314-325, 330-331.

35. Leeuwenhoek to Christopher Wren, 22 January 1683, ibid., IV, 10-11.

36. Leeuwenhoek to the Royal Society, 5 January 1685, ibid., V, 66-67; Leeuwenhoek to the Royal Society, 23 January 1685, ibid., pp. 136-137; Leeuwenhoek to the Royal Society, 30 March 1685, ibid., pp. 154-155, 168-169.

37. Leeuwenhoek to the Royal Society, 30 March 1685, in Leeuwenhoek, A.d. B., V, 198-199. See also Leeuwenhoek to Christopher Wren, 22 January 1683, ibid., IV, 10-13.

38. Leeuwenhoek to the Royal Society, 30 March 1685, ibid., V, 198-201; de Graaf, De mulierum organis, pp. 400401; H. D. Jocelyn and B. P. Setchell, notes to Regnier de Graaf on the Human Reproductive Organs (J. Reproduct. Fert. suppl. no. 17; Oxford, etc.: Blackwell Scientific Publications, 1972), 206n275.

39. Leeuwenhoek to the Royal Society, 30 March 1685, in Leeuwenhoek A.d.B., V, 176-179; Leeuwenhoek to the Royal Society, 13 July 1685, ibid. pp.248-249, 266-267.

40. Leeuwenhoek to the Royal Society, 30 March 1685, ibid., 176-179. See also Leeuwenhoek to the Royal Society, 11 July 1687, ibid., VI, 330-331.

41. Leeuwenhoek to Nehemiah Grew, 25 April 1679, ibid., III, 20-21.

42. Leeuwenhoek's comment, footnoted immediately above, might suggest that he already had in mind the idea of a preformed man in the spermatozoa, although elsewhere he similarly equated the complexity of microscopic animals in general with that of larger animals, including the human body. (Leeuwenhoek to Robert Hooke, 3 March 1682, ibid., pp. 396-397; Leeuwenhoek to Constantijn Huygens, 20 May 1679, ibid., pp. 58-59.) Thus, no implications beyond a simple emphasis on the complexity of the spermatozoa can be ascribed with confidence to the cited passage. In 1683 Leeuwenhoek wrote that the inner body of the spermatozoa acquired the form of a man as "a whole other nature" (een geheel ander wesen) that is already provided with the heart and entrails, "indeed, all the perfection," of a man. (Leeuwenhoek to Christopher Wren, 22 January 1683, ibid., IV, 16-17.) Whether the spermatozoa had all that perfection before assuming their new nature, however, is not clear.

44. Leeuwenhoek explicitly denied that plants mated. (Leeuwenhoek to the Royal Society, 13 July 1685, ibid., pp. 232-235, 246-247; Leeuwenhoek to the Royal Society, 10 June 1686, ibid., VI, 120-121.) On the spreading recognition of plant sexuality in the 1690s and earlier, see Conway Zirkle, introduction to The Anatomy of Plants by Nehemiah Grew, Sources of Science, No. 11 (London: W. Rawlins, 1682; reprint ed., New York: Johnson Reprint Corporation, 1965), pp. xiv-xvi.

45. Leeuwenhoek to the Royal Society, 13 July 1685, in Leeuwenhoek, A.d.B., V, 230-239; Leeuwenheok to the Royal Society, 12 October 1685, ibid., pp. 308-309; Leeuwenhoek to the Royal Society, 10 June 1686, ibid., VI, 120- 121; Leeuwenhoek to the Royal Society, 10 July 1696, in Leeuwenhoek, Sesde Vervolg, p. 279; Leeuwenhoek to the Royal Society, 23 June 1699, in Leeuwenhoek, Svende Vervolg, p. 102; Leeuwenhoek to the Royal Society, 26 February 1703, Leeuwenhoek Letters, Royal Society Library, London, EL.L3.51, fols. 239V-240r. Cf. Leeuwenhoek to Christopher Wren, 16 July 1683, in Leeuwenhoek, A d.B., IV, 66-67.

46. Leeuwenhoek to the Royal Society, 13 July 1685, in Leeuwenhoek, A.d.B., V, 236-237, 262-264.

47. Leeuwenhoek to Gottfried Wilhelm Leibniz, 18 November 1715, in Leeuwenhoek, Send-Brieven, p. 176; Leeuwenhoek to Antonie Heinsius, 1 May 1695, in Leeuwenhoek, Vijfde Vervolg, p. 53; Leeuwenhoek to the Royal Society, 26 February 1703, Leeuwenhoek Letters, Royal Society Library, London, EL.L3.51, fol 234r. Regarding his insistence that later organic forms had to be within the immediately antecedent forms, see also Leeuwenhoek to the Royal Society, 9 June 1699, in Leeuwenhoek, Svende Vervolg, pp. 95-96; Leeuwenhoek to the Royal Society, 23 June 1699, ibid., p. 103; Leeuwenhoek to Hans Sloane, 25 Dec. 1700, ibid., pp. 305-306; Leeuwenhoek to "Hoog Doorlugtige Furst . . . ," 20 April 1702, ibid., p. 450; Leeuwenhoek to the Royal Society, 26 February 1703, Leeuwenhoek Letters, Royal Society Library, London, EL.L3.51, fol. 238v.

48. Leeuwenhoek to Antonie Heinsius, I May 1695, in Leeuwenhoek, Vijfde Vervolg, p. 57; Leeuwenhoek to the Royal Society, 10 July 1696, in Leeuwenhoek, Sesde Vervolg, pp. 278, 280; Leeuwenhoek to the Royal Society, 23 June 1699, in Leeuwenhoek, Sevende Vervolg, p. 103; Leeuwenhoek to the Royal Society, 26 February 1703, Leeuwenhoek Letters, Royal Society Library, London, EL.L3.51, fol. 238v.

49. Leeuwenhoek to Antonie Heinsius, I May 1695, in Leeuwenhoek,Vijfde Vervolg, pp. 53, 57; Leeuwenhoek to the Royal Society, 9 June 1699, in Leeuwenhoek, Sevende Vervolg, pp. 95-96; Leeuwenhoek to the Royal Society, 23 June 1699, ibid., p. 105. See also Leeuwenhoek to the Royal Society, 26 February 1703, Leeuwenhoek Letters, Royal Society Library, London, EL.L3.51, fols. 239V-240r

50. Leeuwenhoek to the Royal Society, 13 July 1685, in Leeuwenhoek, A.d. B., V, 236-237.

51. Leeuwenhoek to Christopher Wren, 22 January 1683, ibid., IV, 16-17; Leeuwenhoek to Hans Sloane, 25 December 1700, in Leeuwenhoek, Sevende Vervolg, p 305

52. Leeuwenhoek to the Royal Society, 9 June 1699, in Leeuwenhoek, Sevende Vervolg, pp. 92-95 (the relevant plate, which has been placed in the body of the wrong letter, is found facing p. 68). Concerning this hoax by Plantade (Dalenpatius), see Cole, Early Theories of Sexual Generation, pp. 68-72. Curiously, Leeuwenhoek did not comment on Nicolaas Hartsoeker's earlier published depiction of what Hartsoeker supposed lay within a human spermatazoon—a tiny human body with an immense faceless head and the future umbilical cord running down the length of the spermatozoon's tail. (Nicolaas Hartsoeker, Essay de dioptrique [Paris: Jean Anisson 1694], pp. 229-230. This work was also translated into Dutch as Proeve der Deurzicht-kunde, trans A. Block [Amsterdam: Jan ten Hoorn, 1699].) Leeuwenhoek did respond to Hartsoeker's claim in this work to have been the first to have discovered spermatozoa (Leeuwenhoek to Herman van Zoelen, 17 December 1698, in Leeuwenhoek, Sevende Vervo1g, p. 63 ff.), and there was no love lost between the two. Hartsoeker's illustration, however, was acknowledged to be pure speculation, making no claim, that is, to have succeeded with the microscope where Leeuwenhoek had so notably failed. But Leeuwenhoek's silence about this prominent illustration suggests that the basic assumption that the human body in some distorted form lay enclosed within the spermatozoa did not clash, after all, with Leeuwenhoek's own beliefs.

Leeuwenhoek also had earlier protested against a published report that attributed to him the idea that the semen was full of tiny infants, but his denial did not deal with what might lie within the spermatozoa. (Leeuwenhoek to the Royal Society, 30 March 1685, in Leeuwenhoek, A.d.B., V, 206-209. See also Steven Blanckaert, Collectanea medico-physica, 2-3 11681-1682; published in Amsterdam: Johan ten Hoorn, 16831, 8; Cornelis Bontekoe, Alle de philosophische, medicinale en chymische Werken Amsterdam: Jan ten Hoorn, 16891,1, 82.)

53. Leeuwenhoek to the Royal Society, 7 September 1688, in Leeuwenhoek, A.d.B., VIII, 10-15.

54. Leeuwenhoek to the Royal Society, 9 June 1699, in Leeuwenhoek, Sevende Vervolg, p. 92.

55. Leeuwenhoek to the Royal Society, 15 April 1701, in Leeuwenhoek, Sevende Vervolg, p. 325.

56. Leeuwenhoek to the Royal Society, 30 March 1685, in Leeuwenhoek, A. d.B. V, 190-191; Leeuwenhoek to the Royal Society, 13 July 1685, ibid., pp. 236-237.

57. Leeuwenhoek to Antonie Heinsius, I May 1695, in Leeuwenhoek, Vijjfde Vervolg, pp. 53-54. Concerning the great secret, see also notes 126 and 127 below, as well as Leeuwenhoek to the Royal Society, 23 June 1699, in Leeuwenhoek, Sevende Vervolg, p. 105.

58. Leeuwenhoek to the Royal Society, 9 June 1699, in Leeuwenhoek, Sevende Vervolg, p. 96.