552 



THE EMBRYOLOGY OF THE FROG 



MATURATION 



It has been stated that by the time the sperm enters the egg, the 

 second polar division has already taken place, or rather, the metaphase 

 of the second division. is in the process of taking place. This division 

 is completed rapidly and cuts off the second polar body in about thirty 

 minutes after the sperm enters the egg. The second polar body is either 

 the same size or smaller than the first. The egg-nucleus then assumes 

 its normal form. The polar bodies are often seen floating about in the 

 perivitelline space. 



The male and female pro-nuclei now move toward the center of the 

 egg and meet in the usual manner. The female pro-nucleus does not 

 leave any pigment in its trail as does the male. The sperm centrosome 

 and centrpsphere divide to form the poles of a small but typical cleavage 

 figure, which is always located toward the animal pole, never in the 

 center of the egg. 



Immediately after fertilization, there is a streaming of the formative 

 protoplasm upward, and the deutoplasm downward, so that the animal 

 pole obtains practically no yolk, and the vegetal pole is composed almost 

 entirely of it. At this time also, the pigment granules directly opposite 

 the point where the sperm enters the egg are carried away, which leaves 

 a somewhat crescent shaped lighter area. This crescentic area extends 

 from half to two-thirds the distance around the egg, and is known as 

 the gray crescent (Fig. 320). This moving of the heavier portion to one 

 side changes the specific gravity of the egg so that the portion possess- 

 ing least weight lies uppermost and close to the gray crescent just oppo- 

 site the point of entry of the sperm. 



Fig. 320. 



Frog's eggs showing formation of gray crescent from side and from vegetal 

 pole. The animal pole is heavily pigmented. 



In about an hour and a half after the entrance of the sperm, in Rana 

 fusca, according to Bracheti, the egg has arranged itself in the manner 

 described, and is now ready for the first cleavage. A vertical plane is 

 drawn through the point where the sperm enters the egg and passes 

 over the top of the egg through the egg-crescent. This becomes the mid- 

 line, on both sides of which the bilateral embryo is to develop. 



There are three distinct substances of varying specific gravity in 

 the frog's egg, namely, protoplasm, pigment, and deutoplasm, and these 



