A story in today’s Manchester Evening News claims that ‘thousands of bird-brained British children’ think that cows lay eggs. Other findings, from a poll by Dairy Farmers of Britain, include the belief that bacon comes from sheep.

A close inspection revealed that things are not as bad as the headlines paint: the survey was of ‘more than’ 1,000 children aged between eight and 15, of whom just 2 per cent – around twenty children – thought that cows laid eggs. Pace extrapolation, hardly the ‘thousands’ claimed in the headline.

The study found that town-bred folks did worse than their country cousins, and it is indeed a matter for regret that many children probably think that fruit and vegetables, if they consume any at all, grow in plastic bags in supermarkets. However, those twenty or so children who said that cows lay eggs needn’t be as bird-brained as they seem. Because, you see, cows do lay eggs, and the discovery that they do so is a cornerstone of biology.

Historically, the Question of Generation – colloquially, How Babies Are Made – has been the single biggest issue in biology. One might even say that biology was, and is, animated by that very issue. How is it that a few minutes of fumbling on a Saturday night can, in the course of a few short months, produce, with disarmingly casual reliability, another human being?

It took Aristotle, rightly regarded as the father of biology, to propose the first credible hypothesis to explain what everyone had hitherto taken for granted. He reasoned that for all their diversity, animals came in two forms – those, like hens, that laid eggs; and those, like cows (and people) that didn’t.

In the course of wondering what happened in non-egg-layers, he asked the kind of simple question that had occurred to no-one else (thus establishing his genius). That is, why do women cease menstruating during pregnancy? What happened to all that unshed blood? Quite reasonably, he supposed that the male ‘seed’ coagulated nine months’ worth of menstrual blood, shaping it into a fetus. Apart from some distasteful and hopefully apocryphal tales of Cleopatra ordering the slaughter and dissection of female slaves in various stages of pregnancy, Aristotle’s hypothesis survived for millennia, untested.

The challenge to Aristotle came more than 1,500 years later, with a book published in 1651 entitled Exercitationes de generatione animalium, the last and (I would argue) greatest work by the English physician William Harvey (1578-1657), famous mainly for his discovery of the circulation of the blood.

Exercitationes was stimulated by a lifelong interest in the Generation Question, and contained real experimental data – dissection of recently mated does from the deer park of his royal patron, King Charles I. Examining the wombs of the dissected animals, Harvey found no trace of fauns coagulating from blood. In fact, he found no trace of anything at all.

This was odd, thought Harvey, who knew that the reproductive tracts of chickens are full of a foam and a fury of business, as eggs do indeed seem to coagulate from a more inchoate mass. What was going on?

Being a good scientist, Harvey believed the evidence of his own eyes rather than the authority of Aristotle, and confessed himself stumped. The Royal Deer, unlike chickens, didn’t lay eggs. But they didn’t do anything else, either. So where did all those cute baby deer come from?

The answer came from follow-up experiments, in which Harvey dissected does that had been mated days or weeks earlier, and discovered, in their wombs, formless, water-filled sacs, never present in unmated deer, but whose origin was obscure. These are now thought to have been early embryos, but in Harvey’s day, this would have been impossible to establish. If insemination occurred days or weeks earlier, any connection between these blobs and the fact of insemination could only be inferred, not proved.

But that’s when Harvey made a grand intuitive leap. These formless primordia, for all their obscurity, were in every way equivalent to the eggs of hens. Leaving aside the view that the distinction between egg and embryo lay decades, if not centuries, in the future, Harvey suggested that eggs and primordia were functionally equivalent, destined to become independent, adult creatures, whether chickens or deer. Aristotle’s old distinction between egg-layers and the rest was found to be spurious: the title pages of early editions of Exercitationes show the enthroned Zeus holding an egg at the point of hatching, an egg from which a parade of beasts is about to emerge. The egg is inscribed Ex Ovo, Omnia – everything comes from the egg.

So perhaps those chicken-headed children know a thing or two, after all.