I’m always surprised at how little the people interested in having as much sex as possible actually know about reproduction and procreation. Much of the history is fascinating stuff, filled with interesting characters who had amazing ideas.
Things that we take for granted today weren’t always so, and by way of a quick primer, I should like to offer this extract from the work of Tim Birkhead (Professor of Behavioural Ecology at the University of Sheffield) who explains this subject far better than I could, and really brings the history to life.
Eggs and sperm – the stuff of occasional miracles and frequent accidents. Although we now take it for granted that sperm and egg must fuse to produce a new being, the road to this discovery was as long and winding as the oviduct itself. This is hardly unexpected given the microscopic size of sex cells and the temporal separation of insemination, fertilization and birth. It is also not surprising that our understanding of male and female roles in reproduction should have fluctuated through the course of time, In Homer’s day females ruled supreme in reproduction and pregnancy was thought to result from microscopic ‘animalculae’ carried in the air which somehow found themselves inside the female. The man’s role was unimportant and the concept of paternity unknown since ‘man lacked all sense of responsibility for survival of the species’1 The very term ‘Mother Nature’ stems from this period in which goddesses were all important and females dominated reproduction. Easter is the pagan legacy of this; named after the goddess Oestrus and celebrated with the ultimate symbols of female fertility – Easter eggs.
In his book Ornithologia, published in 1599, the Italian scholar Ulisse Aldrovandi commented on the great lustfulness of the rooster.2 In contrast to other birds like the eagle and sparrow, who ‘copulate less frequently and are content with a single partner, the rooster treads his numerous wives fifty times a day.’ Aldrovandi also noted that aggression among cockerels was not associated with the acquisition of food or protection of their offspring, but was motivated entirely by the desire to maintain sole control over their females: ‘The rooster fights because he does not wish any of his hens to be touched by anyone and he thus performs the functions of a wise father protecting his honour.’ The cause of the cockerels’ salacity, Aldrovandi suggested, was their ‘especially abundant genital semen: since they cannot endure the irritation it produces they hurry towards sexual satisfaction.’ The Italian anatomist Fabricius ab Aquapendente (1537-1619) was the first to identify the ovary of the hen as the source of ova, but was unable to transpose the concept to humans because the ovaries of birds and women are so different in appearance. Fabricius taught and greatly influenced William Harvey (1578-1657), whose main claim to fame was discovering the circulation of the blood. The two men differed on a number of points relating to reproduction. Fabricius thought that hens could store viable sperm for an entire breeding season (several months) following a single insemination, but Harvey accurately showed that thirty days was the maximum duration.3 Fabricius thought that semen stimulated the generative process without entering the egg; Harvey was convinced that embryonic development was initiated by semen penetrating the egg, but without a microscope he was unable to demonstrate this.
The story of the discovery of spermatozoa – literally ‘sperm animals’ – by Anton Leeuwenhoek (1632-1723), or more likely by his student Johan Ham, is well known.4,3 Leeuwenhoek reported to the Royal Society how, by means of his home-made microscope – comprising a single exquisitely ground lens, which magnified 300 times – he had observed in his own semen millions of vigorously swimming spermatozoa. His letter to the Royal Society is somewhat coy: ‘What I investigate is only what, without sinfully defiling myself, remains as a residue after conjugal coitus. And if your Lordship should consider that these observations may disgust or scandalise the learned, I earnestly beg your Lordship to regard them as private and to publish or destroy them as your Lordship thinks fit.’ Luckily, the Royal Society thought it appropriate to publish Leeuwenhoek’s findings, in which he suggested that is was the minute microscopic creatures swimming in the semen that entered the egg and resulted in fertilization. This was controversial stuff, and some of his colleagues at the Royal Society thought that all that Leeuwenhoek had seen were parasites. After all, Leeuwenhoek had shown the existence of numerous microscopic animals when he examined the scrapings from his teeth!
It wasn’t until a further century had passed that another Italian, Lazzaro Spallanzani, a priest cum scientist, provided unequivocal evidence for Leeuwenhoek’s spermatozoa hypothesis for fertility. Given its current unease with matters sexual, it seems rather surprising that the Church should have provided Spallanzani with both moral protection and financial assistance in his efforts to establish the role of semen in reproduction. Spallanzani worked mainly with frogs, whose reproductive behaviour had been lovingly described by the Dutch biologist Jan Swammerdam (1367-80) in his Book of Nature.3 During the breeding frogs:
“become so eagerly intent on the business of propagation, that they take no care in a manner of their own safety … The male frog leaps upon the female, and when seated on her back, he fastens himself to her … and throws his forelegs round her breast … He most beautifully joins his toes between one another, in the same manner as people do their fingers at prayer … and closes them so firmly that I found it impossible to loosen them with my naked hands … At last the eggs are discharged in the female’s fundament in a long stream, and the male … immediately fecundifies, fertilizes or impregnates them by an effusion of his semen. As soon as these eggs have escaped from the female body, between hers and the male’s hinder legs, and have been impregnated by the male’s semen, the two frogs abandon each other.”
Inspired by some novel but unsuccessful experiments of two colleagues, Spallanzani made pairs of prophylactic oilskin trousers for male frogs to prevent their semen from reaching the females eggs.3 The experiment worked: despite the encumbrance of the trousers the males grasped the females, whose eggs were not fertilized, ‘for want of being bewedded with semen’. Spallanzani then conducted the other essential part of the experiment. Recovering the drops of semen from inside the trousers, he applied these to a female’s eggs which subsequently developed. Moving swiftly from external to internal fertilization and from frogs to dogs, Spallanzani performed the ultimate experiment. He took a female spaniel and before she came on to heat placed her under lock and key inside his apartment, away from male dogs. When she was obviously in oestrus Spallanzani found a male spaniel ‘which furnished me, by spontaneous emission, with nineteen grains of seed, which were immediately inseminated’ into the female. Sixty-two days later ‘the bitch brought forth three very lively puppies’ which resembled both the male and the female. This was the first ever successful artificial insemination involving internal fertilization. With some justification Spallanzani was delighted with his efforts: ‘the success of this experiment gave me a pleasure which I have never experienced in any of my philosophical researches.’
Spallanzani’s studies demonstrated unequivocally for the first time that semen was essential for fertilization, and in doing so dispelled the centuries-old concept of spontaneous generation. Notwithstanding these clever experiments Spallanzani still thought that ‘spermatic worms’ played no role in fertilization. The reason for this was the outcome of another ingenious investigation in which he filtered semen in order to establish which component of semen – sperm or seminal fluid – was responsible for fertilization. A mixture of filtered semen and eggs generated fertile eggs and Spallanzani deduced, entirely logically, that it was the seminal fluid rather than the spermatozoa that triggered development. What he had not realized was quite how difficult it was successfully to separate sperm from seminal fluid and it is now obvious to us that some sperm must have remained. On the basis of these experiments Spallanzani believed that the seminal fluid stimulated the foetal heart, which lay pre-formed inside the egg, and triggered development.5 It was nearly another century before George Newport in 1853 showed, again using frogs, that sperm actually penetrated the egg and were essential if fertilization was to occur.4
Spallanzani was an ‘ovist’, believing each egg to contain a pre-formed embryo. In rather vigorous contrast, the ‘spermists’ thought that the sperm contained the entire embryo and that copulation and insemination were little more than embryo transfer. In the spermist’s scheme the female was regarded merely as a recipient vessel to provide the optimum environment for the embryo’s growth. Nicholas Hartsoeker (1665-1725) encapsulated the spermists view of the male’s central role in reproduction in his drawing of a sperm containing an extremely cramped homunculus with a gigantic head, In fact, Hartsoeker never claimed to have seen the little man inside a sperm, merely that if he could this is what it would look like. Nevertheless, the idea of a pre-formed body inside each sperm was an appealing, and not unreasonable, one during the seventeenth and eighteenth centuries. It did, however, worry James Cooke, an English doctor, who wondered in 1762 what happened to all the sperm that did not give rise to a new person.4 He thought they might not die, but ‘live a latent life, in an insensible or dormant state, like Swallows in Winter, lying quite still like a stopped watch when let down, till [they] are received afresh into some other male Body of the proper kind’.
But it was these wasted sperm, together with their minute size, that finally brought the demise of the spermist viewpoint. The French physician Pierre-Louis Moreau de Maupertius summed up the spermist’s problem in 1744: ‘This little worm, swimming in the seminal fluid, contains an infinity of generations, from father to father. And each [pre-formed creature inside the sperm] has his seminal fluid, full of swimming animals so much smaller than himself.’ Sperm within sperm within sperm … on and on into infinity. Hartsoeker tried to calculate how small the sperm in the original rabbit would have to have been to account for all the rabbits that had ever lived. But it didn’t add up. Or, rather, it did, but the answer was so incredible, a figure involving 100,000 zeros, that it seemed ludicrous.
Exactly the same problem faced the Ovists.3 The mother of us all was Eve and her ovaries must, like a Russian doll, have contained an endless series of smaller and smaller homunculi to sustain the human race. Hardly a likely scenario. There were other objections: the ovist view could not, for example, account for the occurrence of hybrids: if the ovum of a horse contained a pre-formed horse, where do mules come from?
The alternative to the pre-formationist view of both the spermists and the ovists was epigenesis – the idea that embryos resulted from the fusion of male and female sex cells, an idea favoured by William Harvey, among others. But even this had its problems. While the observations of early embryologists were consistent with the generation of new structures arising during development, there had to be some sort of pre-formation to account for the resemblance between offspring and their parents. The transfer of this information, it was deduced, must occur at conception.4 The turning point came in 1875 when Oscar Hertwig showed, using sea urchins, that the sperm head fused with the female genetic material inside the egg to form the nucleus of a new being.
- Jöchle, W., ‘Biology and pathology of reproduction in Greek mythology’, Conception (1971), 4, 1-13
- Lind, L. R., Aldrovandi on Chickens (Norman: University of Oklahoma Press, 1963)
- Pinto-Correia, C., The Ovary of Eve: Egg and Sperm Preformation (Chicago: University of Chicago Press, 1997)
- Moore, J. A., Science as a way of Knowing (Cambridge, Massachusetts: Harvard University Press, 1993)
- Laurila, A., and P. Seppa, ‘Multiple paternity in the common frog (Rana temporaria): genetic evidence from tadpole kin groups’, Biological Journal of the Linnean Society (1998), 63, 221-32