GENERATION OF ANIMALS, III. in. 



rough ; so that the reason why it does not take its 

 young ones in afterwards " is also the reason why it 

 does not produce them alive at the outset : just as 

 the size and roughness of its head prevents them from 

 going in, so also it prevents them from coming out. 

 Since, then, the egg of the Selachia has a soft shell 

 (because they cannot make the envelope hard and 

 solid, being colder creatures than birds are), the egg 

 of the fishing-frog is the only one that is hard and 

 stout, so as to keep it safe in the outside world ; the 

 others' eggs are liquid and soft in nature, because 

 they are inside the mother and get their shelter from 

 her body. 



The process of generation out of the egg is the same Develop- 

 both for the fishing-frogs, which are perfected exter- ^^br/J/''^ 

 nally, and for those Selachia which are perfected 

 internally ; and as between the latter and the birds, 

 it is partly similar, partly dissimilar. First of all, they 

 lack the second umbilical cord which extends to the 

 chorion under the surrounding shell, and the reason 

 for this is that they have not got this shell round 

 them, as it is no use to them, their shelter being pro- 

 vided by the mother ; whereas for eggs that are laid 

 externally the shell is there to act as a protection 

 against injury from without. Secondly, with these, 

 as with birds, the process of generation originates 

 from the extremity of the egg, though not at the 

 place where it is attached to the uterus. A bird's 

 development begins from the pointed end, which is 

 the place where the egg was attached, the reason 

 being that a bird's egg becomes separated from the 

 uterus, whereas the eggs of most, though not all, 



may be the foundation of this remark ; cf. also H.A. 

 565 b 24 ff. 



299 



