The Discovery of Egg and Sperm in the 17th Century - Phenix-Vet

4 M CobbSociety in London (Swammerdam n.d.). Although neitherman had yet provided any clear evidence that humanovaries contained eggs, they were now on a collisioncourse.De Graaf apparently won the race when, in March1672, he published his brilliant De Mulierum OrganisGenerationi Inservientibus Tractatus Novus (New treatiseconcerning the generative organs of women) (De Graaf1672; Jocelyn and Setchell 1972). Although the bookcontained dissections of humans, rabbits, hares, dogs,pigs, sheep and cows, its decisive part was a section onrabbit mating and pregnancy. Here, de Graaf referred tothe follicles or their contents as eggs, and gracefully gavehis former teacher, van Horne, the credit for thisdiscovery, pointing out that Steno had merely said thatovaries contained eggs without identifying them. Aboveall, de Graaf used careful experimental dissection toshow that in rabbits the follicles reddened and rupturedfollowing mating and that 3 days after copulation, smallspherical structures could be found in the Fallopiantubes. De Graaf emphasized that the number of thesespheres was generally identical to the number of rupturedfollicles and that it never exceeded them. Because matinginduces follicular rupture in the rabbit, de Graafmistakenly suggested that the same thing must happenin women, which would imply that virgins would showno ruptured follicles. Finally, like Harvey, de Graaflooked for signs of semen in the uterus and Fallopiantubes, but could find none. He concluded that only a‘seminal vapour’ reached the eggs and fertilized them.Through his experimental work, de Graaf showed thepower of the scientific method, and also demonstratedthat mammals – including women – have eggs.In response, in May 1672, Swammerdam publishedhis own account of human generation, MiraculumNaturae, sive Uteri Muliebris Fabrica (The miracle ofnature, or the structure of the female uterus) (Swammerdam1672a,b). This contained no experimentalstudies, merely a great deal of dissection, some of whichwas highly pertinent, as it showed that virgin womenalso have ruptured follicles. Swammerdam also exploredthe very real problem of how the fluid in the follicle –which he felt was the egg – could move from the ovaryacross into the Fallopian tube. But the central themewas Swammerdam’s claim that van Horne had been thefirst person to accurately describe the female reproductiveorgans and that he, Swammerdam, had firstsuggested that the egg was in the follicle. The bookclosed with a polemical appendix, in which Swammerdamsneered at the quality of de Graaf’s drawings andeven at the fact he had studied ‘rabbits and bruteanimals’ rather than humans, thereby inadvertentlyshowing that he had entirely missed the point of deGraaf’s work. Swammerdam dedicated his book to theRoyal Society, and sent it off to London, followed by abeautifully preserved human uterus, along with twelveother items of genital anatomy, both male and female.Above all, Swammerdam asked the Royal Society toadjudicate on who had the priority in stating thatwomen have eggs.Less than a year later, as the Dutch Republic waswracked by a murderous invasion by the French in theopening battle of a war that lasted 6 years, de Graafproduced a blistering response to Swammerdam, PartiumGenitalium Defensio (Defence of the genital parts)(De Graaf 1673a,b). This savage polemic – written in theimmediate aftermath of the death of de Graaf’s montholdbaby son, Frederick – reproduced letters between deGraaf and Swammerdam, quoted conversations andmade accusations that far exceeded anything seen in amodern row on the internet. De Graaf flatly denied thathe knew that Steno had ever published about eggs (thetwo men had in fact corresponded on the questionseveral times), and accused Swammerdam of plagiarismand of piling ‘lie upon lie’. De Graaf then sent what headmitted was a ‘not very polite’ book to the RoyalSociety and, like Swammerdam, asked them to judgewho had priority.In response to the pleas from the excitable Dutchmen,the Royal Society set up a three-man committee to dealwith the fractious dispute. When the committee eventuallyreported, in October 1673, they decided that neitherde Graaf, nor Swammerdam, nor van Horne had beenfirst to discover that women have eggs. That honour,they stated, went to Steno. This verdict turned out to becompletely pointless. A week before the committeecompleted its work, de Graaf died, aged only 32.Swammerdam wrote an immediate reply, of which notrace remains, and then never referred to the matteragain. Steno, meanwhile, was on the verge of abandoningscience to become a Catholic bishop, and there is noevidence he ever heard of his ‘victory’. Even the RoyalSociety seems to have lost heart, for the report was notpublished for more than 80 years (Birch 1756–7).Ironically, history has adopted a very different view:the follicles are now known as Graafian follicles and thepart played by Steno, Swammerdam and van Horne isforgotten to all but a handful of historians.Whatever the ins and outs of the priority dispute, thekey issue was that by the mid-1670s, the ‘egg theory’came to dominate. In 1679, the French scientificpublication Journal des Sçavans wrote: ‘The view thatman, as well as all other animals, are formed from eggsis something that is now so widespread that there arehardly any new philosophers who do not now accept it’(Anonymous 1679). However, even as the ink wasdrying on the page, an amazing new discovery waschallenging that view, and would once again throw thescientific community into turmoil.Enter the SpermWhen Reinier de Graaf sent his polemical book to theRoyal Society in April 1673, he included a report whicha friend of his, ‘a certain very ingenious person namedLeeuwenhoek, has achieved by means of microscopes’(De Graaf 1673a,b). Leeuwenhoek’s brief letter includeddescriptions of a bee sting, a louse and a moss. This wasthe beginning of a long relationship between Leeuwenhoekand the Royal Society, which spanned the next50 years and consisted of nearly 200 letters from theDutchman (Dobell 1932).Unlike de Graaf, Steno and Swammerdam, Leeuwenhoek(he later adopted the prefix ‘van’ as an affectation)was not academically trained. He was a draper, who hadbegun making his own single-lens microscopes forÓ 2012 Blackwell Verlag GmbH

The Discovery of Egg and Sperm 5reasons that remain obscure (Ford 1985). Although themicroscope had been invented at the beginning of the17th century, they were generally little better thanmagnifying glasses (Ruestow 1995; Wilson 1995). Thepower of this instrument was brought to public attentionin 1665 when Robert Hooke (1635–1703) publishedhis magnificent book Micrographia, complete withstunning illustrations (the full text of this amazing workcan be found on the internet). But Hooke used acompound microscope, which was very difficult to focus,as Samuel Pepys (a member of the Royal Society)discovered when he bought one of these new-fangleddevices having been impressed by Hooke’s book (‘themost ingenious book that I ever read in my life’). To hisdisappointment, Pepys found it very difficult to seeanything like the images printed by Hooke, and he feltthat the princely sum of £5 10s he had spent on themicroscope was wasted (Tomalin 2002).The single-lens microscope was far easier to construct– it simply involved polishing a tiny glass ball approximately1 mm across – and it was widely adopted in theDutch Republic, with some of its main practitionersincluding Swammerdam, who saw red blood cells anddrew the first cell divisions in a fertilized frog egg, andthe great philosopher Benedict Spinoza (Ruestow 1995).Leeuwenhoek’s long career as a pioneer microscopistwas encouraged by Henry Oldenburg, who publishedLeeuwenhoek’s first letter in the Philosophical Transactionsand then, in 1674, wrote that letter to Leeuwenhoek,inviting him to turn his attention to thecomposition of various bodily fluids, including semen(Cole 1930). Over the next few years, Leeuwenhoek senta number of letters to London, amazing his readers withdescriptions of tiny ‘animalcules’ in water (these wereprotists and large bacteria) (Leewenhoecks (sic) (1677).Then, in autumn 1677, a young medical student fromLeiden, Johann Ham, brought Leeuwenhoek some pusmixed with semen to examine. Ham claimed that thishad been produced by a ‘friend’ who had ‘lain with anunclean woman’. When Ham had looked at the sampleunder his microscope, he had noticed many ‘animalcules’in it; alarmed or enthused (it is not clear which),Ham brought the sample to Leeuwenhoek, who confirmedthe observation and did the obvious thing: helooked at his own semen. To his amazement, Leeuwenhoeksaw there were millions of tiny animalculesthrashing about in the sample. These things wereeventually given the name by which they are still knowntoday, and which completely misclassifies them: ‘spermatozoa’– semen animals.Three months later, Leeuwenhoek dispatched a carefullyworded letter to London, written in Latin, insistingthat should the society consider it ‘either disgusting, orlikely to seem offensive to the learned, I earnestly begthat [it] be regarded private and either published orsuppressed as your Lordship’s judgement dictates’(Leeuwenhoek 1678). In a process that will be familiarto modern readers, the Royal Society then sat onLeeuwenhoek’s earth-shattering discovery. Oldenburghad died a few months earlier, and there was a hiatus inthe offices of the Philosophical Transactions. The neweditor, Nehemiah Grew, had his doubts about Leeuwenhoek’sfindings and eventually wrote back in January1678 asking him to look at the semen of dogs, horsesand other animals. Two further letters from Delftfollowed with the requested observations, but theymerely added to Leeuwenhoek’s previous findings. Thelack of urgency suggests that no one at the RoyalSociety understood why Leeuwenhoek’s discovery wasso important (they do not appear to have considered it‘disgusting’). Leeuwenhoek’s letter was finally publishedin January 1679, nearly 18 months after his initialdiscovery (Leeuwenhoek 1678).Leeuwenhoek’s letter makes clear that he thought thatthe spermatozoa were merely another example of the‘animalcules’ he could see everywhere he pointed hismicroscope. He was much more interested in a tangledmass of tiny vessels which he saw in ‘the denser substanceof the semen’. This structure led Leeuwenhoek to claimwithout the slightest evidence that ‘it is exclusively themale semen that forms the foetus and that all the womanmay contribute only serves to receive the semen and feedit’. In other words, Aristotle had been right all along. Theexact nature of the mass of vessels remains obscure –Leeuwenhoek later admitted that it was ‘merely accidental’(Leeuwenhoek 1683). Whatever the case, Leeuwenhoekdid not immediately grasp the significance of hisdiscovery of animalcules in semen.Other people were far more astute than either Leeuwenhoekor the Royal Society, and realized that theanimalcules themselves were highly significant. Prior topublication, news got out about what Leeuwenhoek hadseen. A few months after Leeuwenhoek’s original observation,in January 1678, Swammerdam wrote a letter toThe´venot stating that he had observed ‘innumerable smallworms’ in mouse and dog semen (Lindeboom 1975). Evenmore importantly, in the summer of 1678 – over 6 monthsbefore the Royal Society published Leeuwenhoek’s letter– Huygens printed a brief account of the study in theJournal des Sçavans, which concluded: ‘This latestdiscovery, which has been made in Holland for the firsttime, seems to me to be extremely important and willprovide material for those who seriously study thegeneration of animals’ (Huygens 1678).150 Years of ConfusionA naive modern reader could reasonably assume thatthis was the end of the matter and that everyone soonrealized that egg and sperm were complementary, eachcontaining half of what was necessary to produce newlife. Not at all. Firstly, there were technical issues: noone had yet seen a human egg, and would not do so until1827 (Von Baer 1956). But above all, it was not clearwhat the discoveries meant. For nearly 150 years,thinking about generation was dominated by either‘ovist’ or ‘spermist’ views (Pinto-Correia 1997; Roger1997). Each approach considered that only one of thetwo parental components provided the stuff of whichnew life was made, with the other component was eitherfood (as the spermists saw the egg), or an immaterialforce that merely ‘awoke’ the egg (as the ovists saw thespermatozoa).There were many reasons underlying this apparentscientific dead end. For example, in chickens the twoelements did not seem to be equivalent at all: there was aÓ 2012 Blackwell Verlag GmbH

6 M Cobbsingle, enormous egg which was apparently passive, whilethe ‘spermatic animals’ were microscopic, incrediblyactive and present in mind-boggling numbers. Ultimately,however, the reason why late 17th-century thinkers didnot realize what to us seems blindingly obvious – that bothegg and sperm make equal contributions to the futureoffspring – was that there was no compelling evidence tomake them appreciate this.It was not until the 19th century that the requisitecombination of evidence and theory came together. Tounderstand the complementary nature of egg and sperm,scientists needed to have a theory that could explain thatcomplementarity. This came in two forms in the earlydecades of the 19th century (Cobb 2006b). The developmentof ‘cell theory’ by Schleiden and Schwann gavean explanation for why egg and sperm were equivalent,despite their manifold differences – they were bothreproductive cells. The other factor was that realizationthat heredity had a biological content and that somethingwas inherited, which was contained in egg andsperm, respectively. This development came aboutthrough the conjunction of three areas: the work ofagro-industrialists such as Robert Bakewell, who carriedout massive selective breeding programmes on domesticatedanimals; the studies by thinkers such as Maupertuisand Re´aumur, who explored large and complexfamily trees in the light of particular characters; and byFrench physicians who studied the inheritance ofdiseases (Cobb 2006b). This was finally given form ina monastery in Brno, where Gregor Mendel was just oneof many people thinking about the nature of heredity.Oddly enough, by the time that the fusion of egg andsperm was observed for the first time, by Hertwig andFol in the late 1870s, it was almost an anti-climax.People thought it was obvious. Which in a way, it was,but getting to such a point had been anything butstraightforward, and would not have occurred withoutthat exciting spasm of discovery in the 17th century.Conflicts of interestNone of the authors have any conflicts of interest to declare.ReferencesAnonymous, 1679: Gaspari Bartholini Th.Filii De Ovariis mulierum & generationishistoria Epistola Anatomica. Journal desScavans , 63–64.Birch T, 1756–7: The History of the RoyalSociety of London for Improving ofNatural Knowledge. Millar, London.Cobb M, 2006a: The Egg and Sperm Race:The Seventeenth Century Scientists WhoUnravelled the Secrets of Life, Sex andGrowth. Free Press, London.Cobb M, 2006b: Heredity before genetics: ahistory. Nat Rev Genet 7, 953–958.Cole FJ, 1930: Early Theories of SexualGeneration. Clarendon, Oxford.De Graaf R, 1668: De Vivorum OrganisGenerationi inservientibus de Clysteribuset de usu Siphonis in Anatomia. Hack, Leiden.De Graaf R, 1671: Epistola ad virum clarissimumD. Lucam Schacht. 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Gaasbekios, Leiden.Von Baer KE, 1956: On the genesis of theovum of mammals and of man. Isis47, 117–153.Wilson C, 1995: The Invisible World. PrincetonUniversity Press, Princeton.Author’s address (for correspondence):M Cobb, Faculty of Life Sciences, Universityof Manchester, Oxford Road, ManchesterM13 9PT, UK. E-mail: cobb@manchester.ac.ukÓ 2012 Blackwell Verlag GmbH