MATUIIATION OF THE EGG 107 



water, in which the albuminous investments of the eggs 

 speedily swell up to form the gelatinous mass of the frog's 

 spawn. 



During the discharge of the egg from the ovary, and its 

 passage down the oviduct, further changes occur in its nucleus. 

 The nuclear membrane still further collapses, and finally dis- 

 appears completely; the nuclear fluid and nucleoli become 

 distributed through the substance of the egg, and of the 

 original large nucleus the exceedingly minute nuclear skein 

 alone remains. 



This nuclear skein moves from the centre of the egg to its 

 surface, which it reaches opposite the centre of the black 

 hemisphere. The skein, previously an irregularly tangled 

 thread, now assumes the definite form and arrangement of 

 a nuclear spindle, such as may be seen in the nucleus of an 

 epithelial or other cell immediately before division of the cell 

 occurs. 



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 completion of the act of fertilisation. 



