The Development of the Frog 1 1 



equal parts, and one of these halves is extruaed 

 from the egg and lies under the vitelline mem- 

 brane at the upper or black pole. This ex- 

 truded half of the nucleus is known as the first 

 polar body, and its formation takes place while 

 the egg is still in the oviduct of the frog. 

 Shortly after the egg is laid and the sperm has 

 entered it, the half of the nucleus that remained 

 after the formation of the first polar body again 

 divides into equal parts, and the second polar 

 body is extruded. The part of the nucleus that 

 remains after the formation of the polar bodies 

 is known as the female pronucleus, and con- 

 tains, as may easily be understood, just one- 

 fourth of the material of the original egg 

 nucleus. The two small round polar bodies, 

 lying side by side under the vitelline membrane 

 at the dark pole of the egg, take no further 

 part in the development of the egg, and eventu- 

 ally disappear. 



What the purpose of the maturation of the 

 egg-cell may be it is not possible at the present 

 time to say ; and so many theories on the sub- 

 ject have been advanced that it is very difficult 

 to give any simple statement of the case. 



The formation of the polar bodies or glob- 

 ules is the result of a form of cell division 



