100 THE FROG. 



tozoon, or male pronucleus ; it penetrates further into the egg, 

 carrying the pigment with it, and soon meets the female pro- 

 nucleus, or part of the nuclear skein which remains within the 

 egg after extrusion of the two polar bodies. 



The two pronuclei come into close contact with each other, 

 and, after having increased considerably in size, fuse together to 

 form the segmentation nucle us. This fusion, which occurs about 

 two and a half hours after the spermatozoon first entered the 

 egg, completes the act of fertilisation. 



Almost immediately after the spermatozoon enters the egg a 

 considerable extrusion of peri-yitelline fluid takes place, between 

 the egg and the vitelline membrane (Figs. 45, C and D). 

 This separates the egg from the vitelline membrane, and greatly 

 facilitates the rotation of the egg within the membrane ; from 

 this time, iu whatever position the spawn be placed, the black 

 poles of the eggs will always, from their less specific gravity, be 

 uppermost, and the white poles, which are of higher specific 

 gravity owing to the greater abundance of yolk-granules in them, 

 will be undermost. The extrusion of the peri-vitelline fluid, and 

 the consequent separation of the egg from the vitelline membrane, 

 may possibly serve further to prevent or hinder the entrance of 

 a second spermatozoon. 



THE EARLY STAGES OF DEVELOPMENT OF 

 THE FEOG'S EGG. 



1, _Segmentation of the Egg,, 



Segmentation of the frog's egg is, like that of Amphioxus, a 

 process of cell-division ; but although the processes in the two 

 animals are essentially similar, there are important differences 

 in detail, due to the much larger amount of food-yolk present 

 in the egg of the frog, and its unequal distribution. 



Pood-yolk consists of small granules of highly nutritious 

 matter, imbedded in the substance of the egg ; but although it 

 forms a store of readily assimilated nutriment, at the expense of 

 which the development of the embryo can be effected, it must 

 be remembered that until it has been so assimilated the yolk 

 granules will be foreign bodies, and, like any other foreign bodies, 

 will be a hindrance rather than an aid to development. The direct 

 influence of food-yolk is to mechanically impede the activity of 



