THE DEVELOPMENT OF A MOLLUSK 35 



particular significance in the preparation of the eggs for fertiHza- 

 tion. The process is known as the "maturation" of the egg. 



The sperm cells are inclosed with the ova in the capsules. 

 They consist chiefly of a nucleus with a very insignificant amount 

 of protoplasmic substance. A single sperm cell enters the ovum 

 somewhere about the vegetative pole, at the time of the beginning 

 of the maturation process, and its nucleus gradually makes its 

 way upward toward the egg nucleus, until the two nuclei are in 

 contact. These nuclei, known as the "pronuclei" of the egg, 

 may be seen in Model i,B lying close together in the protoplasm 

 at the animal pole. The egg is now fertilized and capable of 

 developing into a new organism. 



MODEL 1, B 



The fertilized egg, showing the egg and sperm nuclei in contact at the animal 

 pole. On either side of them are the centrospheres. 



Each nucleus is composed largely of a peculiar substance, 

 which has been given the name "chromatin," because of the 

 readiness with which it assumes the stains used for coloring 

 microscopic objects. Though little is known about the definite 

 function and properties of chromatin, its importance is evidently 

 very great, for it is found in the nuclei of all cells. Generally it is 



[II] 



