GENERATION OF ANIMALS, II. iv. 



nature " of milk and the menstrual fluid is one and 

 the same, the action of the semen upon the substance 

 of the menstrual fluid is the same as that of rennet 

 upon milk. Thus when the " setting " is effected, 

 i.e., when the bulky portion " sets," the fluid portion 

 comes off ; and as the earthy portion solidifies mem- 

 branes form all round its outer sui'face. (This is the 

 result of necessity ; but also it is to serve a purpose : 

 (a) Necessity ordains that the extreme surface of a 

 thing should sohdify when heated as well as when 

 cooled ; (b) it is requisite that the young animal 

 should not be situated in fluid but well away from it.) 

 Some of these are called membranes ; some choria ^ : 

 and they differ by the " more and less." '^ They are 

 found in Ovipara and Vivipara aUke. 



Once the fetation has " set," it behaves Uke seeds (6) The 

 sown in the ground. The first principle (of growth) nfeTit of 

 is present in the seeds themselves too, and as soon '^^ embryo, 

 as this, which at first was present potentially, has 

 become distinct, a shoot and a root are thrown 

 out from it, the root being the channel by which 

 nourishment is obtained, for of course the plant 

 needs material for grovrth. So too in the fetation, 

 in a way all the parts are present potentially, but 

 the first principle has made the most headway, 

 and on that account the first to become distinct 

 in actuality is the heart. This is plain not only to Heart. 

 the senses (for after all it is a matter of fact), but 

 also to the reason. Once the fetation which has 

 been formed is separate and distinct from both the 

 parents, it must manage for itself, just like a son who 

 has set up a house of his own independently of his 



» See also H.A. Bk. VI, ch. 3. 

 ' See 737 b 7, n., and Introd. § 70. 



H 193 



