560 SIMPLEST FOKM O7 GENERATIVE PROCESS. 



termed the yolk-bag. The yolk-bag and its contents, namely 

 the yolk and the germinal vesicle, constitute the ovum. The 

 eggs of many animals (as of Birds) contain an additional 

 store of liquid albumen, the " white," enveloping the yolk-bag 

 and destined to be gradually drawn into" it, so as to replace 

 the albumen of the yolk as it is progressively used-up in the 

 development of the embryo ; and the " shell " is a subsequent 

 formation around this ( 755). 



734. The Hydra presents us with a very apposite illustra- 

 tion of the simplest mode in which the generative function is 

 performed. Sperm-cells are developed at certain periods in 

 the substance of its body near the origin of the arms ; whilst 

 ovules are evolved in the wall of the stomach nearer to the 

 foot or base. By the rupture of the sperm-cells, their con- 

 tained spermatozoids are set free in the surrounding water, 

 and they penetrate to the ovules, which are exposed to their 

 influence by the thinning-awa'y of their exterior covering. 

 From what has been observed in higher animals, there seems 

 no reasonable doubt that the spermatozoids make their way 

 through the germinal membrane, and penetrate into absolute 

 contact with the germinal vesicle, which then lies near the 

 surface of the ovule. What is the precise change effected by 

 fertilization, has not yet been fully ascertained ; the germinal 

 vesicle, however, disappears ; and it would seem as if its 

 contents, with the product of the liquefaction of the sperma- 

 tozoids, were diffused through the yolk, which soon begins to 

 undergo changes of a very remarkable nature. 



735. Before going further, it may be well to notice the 

 remarkable antagonism which exists between the processes of 

 Gemmation and Generation, as regards the conditions by which 

 they are respectively favoured. For we see that in the Hydra, 

 as in Plants, the extension of the body into buds is promoted 

 by warmth and a copious supply of food ; so that, as it would 

 appear, if these be afforded, this mode of multiplication may 

 be protracted indefinitely. On the other hand, if the supply 

 of food be limited and the temperature lowered, the pro- 

 duction of buds ceases, and the formation of sperm-cells and 

 of germ-cells begins. The result is that ova or eggs are 

 produced, which have a very firm horny covering, and possess 

 a great power of resistance to cold; and thus provision is 

 made for the continuance of the race through a winter tern- 



