116 DEVELOPMENT OF THE FROG. 



The First Polar Body. About the time the egg is laid, but 

 before it is fertilised, the egg becomes slightly flattened at its 

 upper or black pole, a certain amount of fluid being exuded 

 between the egg and the vitelline membrane. The nuclear 

 spindle now divides into two equal parts, one of which remains 

 within the egg, while the other is extruded from it as the 

 first polar body, a minute ovoidal white globule, which lies on 

 the surface of the egg in the exuded peri-vitelline fluid. 



The Second Polar Body. The half of the nuclear spindle 

 that remains within the egg retreats from the surface a little 

 distance, and then divides into two equal parts, one of which 

 remains within the egg as the female pronucleus, while the 

 other is extruded as the second polar body, a minute white 

 globule very similar to the first polar body, and like this lying 

 freely in the perivitelline fluid on the top of the egg. 



In the case of most animals in which the formation of polar' 

 bodies has been observed, both the first and second polar bodies 

 are extruded before fertilisation is effected. In the frog the 

 extrusion of the second polar body does not occur until after 

 the spermatozoon has entered the egg, though before the com- 

 pletion of the act of fertilisation. 



C. Fertilisation of the Egg. 



Fertilisation, or impregnation, consists in fusion of the sper- 

 matozoon with the egg ; or, more strictly speaking, fusion of 

 the nuclei of these two bodies. 



The spermatozoa, after being shed over the spawn by the 

 male, swim actively by means of their long tails, penetrate the 

 gelatinous investment of the eggs, bore their way through the 

 vitelline membrane, and so penetrate into the eggs themselves, 

 which they enter at or close to the upper or black pole. 



A single spermatozoon is sufficient to fertilise an egg, and it 

 is doubtful whether more than one is ever concerned in the 

 process. 



In about an hour after the spermatozoon has entered, a pig- 

 mented process may be seen projecting inwards from the surface 

 of the egg, with a clear spot in its centre. This spot is the 

 nucleus of the spermatozoon, and is spoken of as the male pro- 

 nucleus : it penetrates further into the egg, carrying the pigment 

 with it, so that it appears surrounded by a pigmented capsule ; 

 connected with the surface of the egg by a pigmented stalk. 



