Everything from the egg. For the Aristotelian William Harvey, the egg was not the structure we know today but a product of conception. The frontispiece of his treatise on generation shows Zeus opening a bisected egg labelled Ex ovo omnia: ‘Everything from the egg’. This releases humans, other animals and plants and so drew on earlier representations of Pandora’s mythical box, from which all good and evil had flown out into the world. Several decades later, the Dutch anatomist Regnier de Graaf postulated that mammals always develop from a female egg. Yet by modern standards what he saw under the microscope was not the egg itself but a cluster of cells around it, the ‘Graafian follicle’. The human egg remained elusive for centuries to come.

Debates over generation

Mechanical philosophers in the 1600s built on Aristotelian epigenesis to explain generation entirely in terms of moving particles and attractive forces, but because this risked charges of atheism for dispensing with God, a new theory of pre-existence soon became dominant.

The theory of pre-existence had it that all adult structures were already present in the egg, only much smaller. God had generated every germ at the Creation, one within the other like a Russian doll. The related doctrine of preformationism argued that the body of the new being was complete in the parent seed so that during gestation the embryo only increased in size. ‘Ovists’ placed the germ in the egg, ‘animalculists’ in the sperm.

During the 1700s epigenesists and preformationists wielded the microscope to support their claims, but could not agree on what they saw or how to interpret it. The controversy represented deeper conflicts: materialism versus orthodox Christianity and empiricism against rationalism. Epigenesis seemingly triumphed, but in fact the rules of the game changed. Embryologists no longer sought to explain the source of organization; they took organization as given and dissected animals to establish its laws.

‘A child in the egg’, 1685


‘The homunculus in the sperm’, 1694