72 EMBRYOLOGY IN ANTIQUITY [pt. ii 



skin, again, is formed by the drying of the flesh, Hke the scum upon 

 boiled substances; it is so formed not only because it is upon the 

 outside, but also because what is glutinous, being unable to evaporate, 

 remains on the surface". Here is a splendid collection of mechanical 

 processes, but Aristotle is careful to add: "As we said, all these things 

 must be understood to be formed in one sense of Necessity, but in 

 another sense not of Necessity but for a Final Cause". 



Concurrent growth and differentiation, the former being temporally 

 sequent to the latter, he thus describes: "The upper half of the body, 

 then, is first marked out in the order of development ; as time goes 

 on the lower also reaches its full size in the sanguinea. All the parts 

 are first marked out in their outlines and acquire later on their colour 

 and softness or hardness, exactly as if Nature were a painter producing 

 a work of art, for painters too first sketch in the animal with lines 

 and only after that put in the colours". Aristotle had some difficulty 

 about the eyes; he noted that they were disproportionately large 

 in early bird embryos, but he seems to have thought that they shrunk 

 absolutely as well as relatively during further development. It takes 

 him a great deal of ingenuity to invent a teleological explanation for 

 this quite imaginary fact. 



The food which the embryo derives from the mother, according 

 to Aristotle, is of two distinct kinds, nutritious, formative, or creative, 

 TO OpeTTTLKov, and that which is concerned with simple increase of 

 size, TO et? fxeyedo'; itolovv tijv eirihoaiv. This distinction is difficult to 

 understand, and, though it would be attractive to interpret the former 

 as vitamines and the latter as fats, proteins and carbohydrates, that 

 would probably be putting too much of a strain on our belief in 

 Aristotle's insight. He has much to say of the placenta, and ascribes 

 to it its correct function. He combats the idea that foetal nutrition 

 is maintained by uterine paps, alleging against it the fact that all 

 embryos are enclosed in membranes. He discusses birds' eggs in 

 great detail, referring to infertile or "wind-eggs" and to the action 

 of heat during incubation. He considered that the embryo was 

 formed from the white exclusively, and only got its nourishment 

 from the yolk, which was a backward step in view of what had already 

 been said by the Hippocratic embryologist. He knew of the whiteness 

 of the yolk when first formed in the oviduct and of the yellow colour 

 of the layers of yolk added to it in its passage down that tube, but 

 he held that the yellow colour was "sanguineous", and therefore hot, 



