As the effluvia of Spring stir reluctantly into glutinous life, we at the Maison Des Girrafes hop hope that Mr and Mrs Jeremy Fisher will return to our wildlife pond and Do Their Thing, as they did last year.

Mr (or, as it may be, Mrs) Jeremy Fisher. Last year

While filled with the happy thoughts of impending rumpo, anuran style, a press-release hit my inbox (HT Mrs Gee) with the following headline

BRITAIN’S FIRST LOVE HOTEL FOR FROGS

And (in part) the following rubric:

1. To mark the frog mating season and the start of the national CSV Action Earth campaign on 3rd March, MSP Ian McKee will join volunteers at Redhall Walled Garden (Edinburgh) to create Britain’s first frog hotel – in a bid to encourage frogs to breed, provide a secure environment and prevent the worldwide decline of the species. Frogs, which are especially threatened by predators during the mating season in March will be given a special ‘love’ hotel made by Redhill volunteers, ensuring they are protected from predators during the mating season and beyond. The two tier frog hotel for ‘love’ will be constructed from recycled materials, sympathetic to the local environment, including a bog garden, terracotta pots, pipes and compost. The frog ‘love’ hotel will have signage advertising ‘dark, damp rooms for rent for hour, day or week’ but will not have a ‘croak-room’.

The Love Hotel for Frogs is a project run by Community Service Volunteers as part of their Action Earth Campaign, supported by the supermarket Morrisons, which involves giving 900 small cash grants to ‘kick-start’ community projects.

Two particular features of the courtship of frogs have long endeared them to researchers interested in the majesty of reproduction and the mystery of development. First, fertilization is entirely external. Dissection and general fossicking around inside dead animals is not necessary. The female lays the unfertilized eggs, and then the male comes along and sprays semen on them. Second, as every schoolchild knows, development in the transparent spawn is very easy to observe.

Frog sex played a pivotal role in one of the most famous scientific experiments of all time, by one of my scientific heroes, Lazzaro Spallanzani (1729-1799). Spallanzani wanted to test whether semen had to come into direct contact with eggs in order to ‘fecundate’ them – that is, to spur them into development. At the time, it wasn’t known whether semen had to come into direct contact with eggs, or if it was sufficient that semen was somewhere in the general vicinity, exuding a kind of spiritous vapour (known as the aura seminalis) that did the job.

Spallanzani established that direct contact was indeed essential. He discovered this by showing that frog development was impeded if the males were engaged in courtship while wearing little taffeta trousers of Spallanzani’s own design. In addition, Spallanzani showed that spawn would develop when exposed to frog semen (trapped in his frog trousers) diluted to various degrees. When a mixture of semen and water was progressively filtered to remove particulate matter, the filtered fluid became less and less potent. If, however, the sediment on the filter was resuspended in the water, the mixture was as potent as ever. You can find the full story in a well-known exegesis¹.

Now were you Spallanzani and had carried out this experiment, you’d probably assume that what was going on was that the particulate matter in the semen had an active role in fecundating the eggs, and you’d be right. The particulate matter contained sperm, which had been known since 1678, when Christiaan Huyghens announced the discovery of creatures found in semen that were

1. formed of a transparent substance, their movements are very brisk, and their shape is similar to that of frogs before their limbs are formed. The discovery, which was made in Holland for the first time, seems very important, and should give employment to those interested in the generation of animals²

Spallanzani, however, looked at his results and came to a completely different conclusion. At that time, the prevailing view was preformationist – that is, that eggs contained all subsequent generations of animals wrapped up inside them, down to an infinitesimal smallness, and that all semen did was to switch on the developmental program that would ‘evolve’ (that is, ‘unfold’) the next adult stage.

This seems entirely loopy to us now, but was – and I can’t emphasize this enough – perfectly consistent with the scientific observation of the time.

Something we now take for granted, to the extent that it is ingrained in our scientific worldview, is the cell theory, the idea that organisms are made of lots of small, individual and fundamental units called cells, some of which act with a fair degree of autonomy. It was the advent of cell theory that finally put paid to preformationism – that an infinite number of future generations were wrapped up tight in the egg. In Spallanzani’s time, however, cell theory lay far in the future.

Before the cell theory, and even before usable microscopes, there was no qualitative distinction between eggs and embryos. When William Harvey found the implanted sacs of embryo deer in the mated hinds in the deer parks of his rotal patron, King Charles I (and wrote up his observations in his 1651 treatise, Exercitationes de generatione animalium_) he assumed that they were just like hens’ eggs, observed in the act of formation in the chook reproductive tract. This was understandable given that Harvey had studied in Padua under Hieronymus FabriciusFabricius, whose work on chickens suggested that given the exceeding complexity of the chicken reproductive tract (and the fact that eggs have hard shells), semen would have a hard time coming into direct contact with the egg. From this and other observations came the idea that semen had to exude some kind of ‘aura’ to effect fecundation remotely, and this is the idea that Spallanzani sought to investigate in frogs.

Crucial to Harvey’s pre-microscopic observations was his finding that nothing was observable in the wombs of hinds that had only just mated – the ‘eggs’ only appeared a little while later. This finding, of course, supports the aura seminalis idea, the notion that semen fecundated eggs without coming into actual contact with them.

However, we now suspect that what Harvey observed weren’t eggs (which would in any case have been too small to see with the naked eye) but embryos in their implanted amniotic sacs: in this light, Harvey’s famous doctrine that Ex Ovo, Omnia – everything comes from the egg – seems misleading. Harvey’s aim, at the time, which seems entirely foreign to us nowadays, was to test Aristotle’s notion that there was a fundamental distinction between animals that produced eggs and others, being viviparous, that appeared not to do so.

Because of this fundamental difference in worldview between then and now, two eras separated by the cell theory, when microscopists first turned their new instruments on living matter, they didn’t immediately say things like ‘Ah! Organisms are made of lots of small, individual and fundamental units called cells!’ What they did do was marvel at the previously unknown structure of very small things, and having done so, had no reason to think that this divisibility couldn’t be applied without limit. If we see all this complexity in living matter, a complexity previously unsuspected, they might have said (with some justification), why should there not be strata of more minute complexity beyond the resolution of scientific instrumentation? In other words, microscopical observations, rather than exploding preformation, actually substantiated it – the idea that future generations might be folded up inside eggs, relying on sperm to set in train the process that unwrapped each layer in its turn.

This idea of complexity and divisibility applied to eggs – which were not seen as distinct from embryos (and which were not recognized formally as single-celled entities until 1828) – and also to sperm. When sperm were first observed, they were not immediately regarded as cells that had anything in particular to do with reproduction, but as – now get this – parasitic worms, with all the integrity to which fully fledged animals are entitled. In an age in which the stench of putrefaction was everywhere, and in which surgeons knew that practically every creature was a bag of parasitic worms, it was easy conclusion to make – that sperm (or ‘spermatic worms’) were parasites that infected the male reproductive organs and were transmitted vertically with semen.

Had Spallanzani actually watched a sperm enter an ovum, then, he wouldn’t have concluded that he’d discovered fertilization – only that spermatic worms are transmitted very early on in the cycle of fecundation in which semen somehow sparked eggs into life. ‘It is rather alarming,’ comments Elizabeth Gasking³

1. to think that had Spallanzani really seen the penetration of the eggs by the spermatozoa he would have regarded it as confirmation of this hypothesis.

True, people did wonder whether sperm had a more direct role in reproduction (perhaps even containing all generations rolled up in their heads, a competing idea to egg-based preformationism) but the more obvious association with parasitism made people uncomfortable – the theory was just too complicated, and as we are all schooled, the more complicated a theory is, the less likely it is to be correct.

Spallanzani was the last great thinker of preformationism. He died in 1799, and preformation, it is fair to say, died with him. When, just a few years later, Dalton came up with his atomic theory, microscopists were infected by the idea that matter, including perhaps living matter, was particulate, and there might be a lower limit to size and complexity. Microcosmos could not indefinitely give way to further microcosmos. It was only then that the idea of eggs and sperm as cells, qualitatively distinct from (but very much a part of) the organisms that contained them could emerge, and our modern conception (cough) of reproduction could be born.

Meanwhile, in the Maison Des Girrafes, the chooks continue to lay their eggs, accompanied by Beelzebun Demon Bunny of DOOM, our egg-laying rabbit, and – we hope – Mr and Mrs Jeremy Fisher might be sufficiently attracted by their own ‘love hotel’ (a handy cave made of a broken flower pot, placed at the water’s edge) to engage in their immemorial fecundation.

¹ Jacob’s Ladder: The History of the Human Genome, by Henry Gee and a Gentleman.

² Quoted on p. 48 of Needham, J. A History of Embryology, Cambridge, 1934.

³ Quoted on p. 43 of Gasking, E. B. Investigations into Generation 1651-1828, London: Hutchinson, 1967.