560 SIMPLEST FORM OF GENERATIVE PROCESS, 
termed the yolk-bag. The yolk-bag and its contents, namel) 
the yolk and the germinal vesicle, constitute the ovum. Th 
eggs of many animals (as of Birds) contain an additiona 
store of liquid albumen, the “ white,” enveloping the yolk-ba; 
and destined to be, gradually drawn into it, se as to replac 
the albumen of the yolk as it is progressively used-up in th 
development of the embryo ; and the “shell” is a subsequen’ 
formation around this (§ 755). 
734. The Hydra presents us with a very apposite illustra 
tion of the simplest mode in which the generative function i 
performed. Sperm-cells are developed at certain periods ir 
the substance of its body near the origin of the arms ; whils 
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 thei 
influence by the thinning-away 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 suppl 
of food be limited and the temperature lowered, the prq 
duction of buds ceases, and the formation of sperm-cells 
of germ-cells begins. The result is that ova or eggs 
_ produced, which have a very firm horny covering, and 
a great power of resistance to cold; and thus provision : 
made for the continuance of the race through a winter te 
