Several barely intelligible accounts of animal reproduction (and in particular of the origins and development of the human embryo) are preserved amongst the fragments of the Presocratic philosophers. It is debatable whether or not these accounts—amongst which one should include zoogonies and anthropogonies such as that of Anaximander who argued that the first living creatures had their origin in a kind of earthy moisture (DK 12 A 30)—should be described as ‘embryological’ in a modern scientific sense. (Anaximander seems mainly to have been concerned with explaining the ultimate origin of man, given that he is unusual in requiring intensive nursing after birth. He ended up by positing a first generation of humans born at puberty.) It should also be remembered that the word embryon in Greek does not always correspond to the modern ‘embryo’, but can often refer even to newly born infants. Ancient ‘embryology’, then, covers a whole range of problems, from generation to the nutrition of neonates. The three Hippocratic treatises (see hippocrates (2)) concerned with generation, heredity and sex differentiation, and paediatric physiology—Diseases 4, On Generation, and On the Nature of the Child together with the later medical works spawned by them, including Soranus' Gynaecology, and Galen's On the Seed—could all be described as wholly or partly ‘embryological’.

Certain basic problems occupied investigators throughout antiquity. In general terms the physiology of conception was central, but much effort was expended on accounting for the sex of the unborn child, the way in which it ultimately assumes the characteristics of its parents, how long all this takes to happen, and whether or not this gestation-period is the same for male and female. There was also a fierce debate, both moral and scientific, over whether the embryo is alive or not, and if so, what kind of life it possesses. Abortion was a controversial practice, as the very word embryosphaktēs, ‘embryo-slaughterer’, used to describe an instrument employed by Soranus of Ephesus, suggests.

Parmenides of Elea seems to have held that sex is dependent on the position of the foetus in the womb—‘on the right, boys; on the left, girls’ (DK 29 B 17, a one-line fragment preserved by Galen); Anaxagoras argued that the side from which the male seed is secreted is important (males on the right, females on the left (DK 59 A 107)). Other theories of sex-determination appealed to the role of heat. Empedocles linked male to hot, female to cold conditions in the womb at the time of conception (DK 31 B 65), or the relative strength of the male and female seed secreted by the parents. Some theories appeal to other types of causal factor—the 5th-cent. bce Pythagorean, Philolaus of Croton, seems to have argued (more judiciously perhaps) that all living bodies are composed of the hot (DK 44 A 27).

Adherents of what is often (but anachronistically) called the ‘pangenesis’ doctrine held that the seed comes from the whole of the body; hence disabled or mutilated parents should be expected to have disabled offspring (see deformity). A father with a mark on his arm will have children similarly marked. This view is generally traced back to Democritus of Abdera, but similar views are expressed in the Hippocratic corpus, notably at Diseases 4. 32 and On Generation 3. Aristotle attacked this type of theory, especially at GA 721^b11 ff. He pointed out—amongst many other objections—that it is simply not true that deformed parents necessarily have deformed offspring. He also argued by analogy with plants that new growth can be produced from cuttings. His own view was that the man's seed provides the form, and the efficient cause of the generation of the embryo, with the woman providing the material. For Aristotle, the heart develops first; the embryo then progressively actualizes its higher and higher potentialities (Parts of Animals 666^a18 ff). (Much ancient work in this area is coloured by the assumed inferiority of the female sex, although some authorities, including Alcmaeon (2), Anaxagoras, and Parmenides, argue that the female secretes a seed which has its own role in reproduction.)

The nature of embryonic development within the womb also attracted attention. How does it grow? (Perhaps the earliest practical and systematic examination of the phenomenon survives in the Hippocratic work On the Nature of the Child 29; here, by drawing an analogy with what he has seen to be the case with developing hens' eggs, the author seeks to demonstrate that the human embryo grows gradually in the womb, surrounded by membranes with an umbilicus at the centre. In the 2nd cent. ce, Galen investigated the anatomy of the human embryo, drawing important conclusions about the difference between the structure of the heart in the unborn and post-natal child.) Or is the embryo ‘concocted’? The doxographers ask: Is the newly conceived embryo a miniature animal, fully formed from the start? Or are new parts added to it as it grows? If the latter, how long does it take for articulations to develop? It is far from clear exactly when the Greeks decided how long the human embryo takes to gestate. Several Hippocratic (and later, Galenic) works deal with the viability of premature babies (On the Seven-Months Child, On the Eight-Months Child). The Hippocratic treatise On Fleshes deals with the problems of the gestation-period in man by assigning special status to the number seven. Whatever parts the body needs, it has them within seven days of the seed entering the womb, argued its author [Fleshes 19]. Numerological speculation remains characteristic of this area of study; its most extreme expression can perhaps be seen in the Hippocratic treatise On Sevens, and ultimately in works with much more general application like the Tetrabiblos of Ptolemy (4). See gynaecology.