GENERATION OF ANIMALS, III. ni. 



fishes of this class remain attached to the uterus 

 even when they are perfect. As the young animal 

 develops at the extremity, the egg gets used up 

 (just as in the case of birds and the other eggs that 

 have been released from the uterus), and at the 

 final stage, by which the animal has reached its 

 perfect development, the umbilical cord remains 

 attached to the uterus." The like ^ holds good in 

 the case of those Selachia whose eggs have been re- 

 leased from the uterus, there being some whose egg 

 is released as soon as it is perfected.*^ 



In view of what has been said, the puzzle may be Differences 

 raised why the processes of generation in birds and gj^^^l^an^" 

 fishes differ in this respect. The reason is that in Seiachia. 

 birds' eggs the white and the yolk are separate, 

 whereas fishes' eggs are single-coloured, the contents 

 being mixed up together throughout, so that there 

 is nothing to prevent the " principle " in them being 

 at the opposite end ; the egg is of similar composi- 

 tion both at the end where it is fastened and at the 

 opposite end, and it is easy for it to draw the nourish- 

 ment out of the uterus by means of passages which 

 lead from this principle. This can clearly be seen in 

 those eggs which do not get released, for in the case 

 of some of the Selachia the egg does not get released 

 from the uterus, but remains connected as it proceeds 

 downwards to produce the young alive. In these 

 cases, the young animal, after it has reached its 

 perfect development, retains its umbilical cord joined 

 to the uterus when the egg has been consumed. Thus 

 it is plain that during the earher stages also, while 



ceding. He means the embryo develops at the extremity. 

 The process Ls similar (" Hke "), not identical. 



' That of the '* fishing-frog ; but see 754 a 26, n." 



301 



