70 EMBRYOLOGY IN ANTIQUITY [pt. ii 



they are possessed actually". These passages show very clearly the 

 line of thought contained in the recapitulation theory, as do the 

 following. "For an animal does not become at the same time an 

 animal and a man or a horse or any other particular animal", i.e. 

 the more general appears first and the more particular later. "For 

 the end is developed last, and the peculiar character of the species 

 is the end of the generation in each individual", i.e. the embryo 

 attains the point of being definitely not a plant before it attains that 

 of being definitely not a mollusc but a horse or a man. Aristotle 

 concludes that the diflferent sorts of souls enter the embryo at different 

 stages of development, just as the shape of the embryo gradually 

 approximates to whatever adult shape it is destined to conform to. 



Aristotle continues to discuss the central problems of embryology, 

 but now in a way which presents features of directly physico-chemical 

 interest. "When the material secreted by the female in the uterus 

 has been fixed by the semen of the male (this acts in the same way 

 as rennet acts upon milk, for rennet is a kind of milk containing vital 

 heat, which brings into one mass and fixes the similar material, and 

 the relation of the semen to the catamenia is the same, milk and the 

 catamenia being of the same nature) , when, I say, the more solid 

 part comes together, the liquid is separated off from it, and as the 

 earthy parts solidify, membranes form all round it; this is both a 

 necessary result and for a final cause, the former because the surface 

 of the mass must solidify on heating as well as on cooling, the latter 

 because the foetus must not be in a liquid but separated from it." 

 Later on, he also says, "The reason is similar to that of the growth 

 of yeast, for yeast also grows great from a small beginning as the more 

 sohd part liquefies and the liquid is aerated. This is effected in 

 animals by the nature of the vital heat, in yeasts by the heat of the 

 juice contained in them". 



These remarkable passages contain the first reference to enzyme 

 action ever made in a discussion on embryology. The solidification of 

 the outer crust is of course Hippocratic, as we have already seen. The 

 part about the amnios is unfortunate ; for the facts are exactly contrary. 



"The heart is first differentiated", says Aristotle, "as is clear not 

 only to the senses (for it is so) but on theoretical grounds. For when- 

 ever the young animal has been separated from both parents it must 

 be able to manage itself, like a son who has set up house away from 

 his father." This is good observation. "The heart is the principle 



