REPRODUCTION 



close together and the protoplasm, or the nucleus, of the 

 one passes into the protoplasm of the other. The fusion 

 that follows gives the cells an increased impetus toward 

 division and the necessary strength for it (Fig. 52). 



In Many-Celled Animals. — Conjugation is not unlike 

 the process of fertilization that occurs in human beings 

 and other many-celled animals. In this process two 

 dissimilar cells unite, and the union results in the 

 formation of a new cell with the power to divide and 

 form other cells. In many-celled animals the cells do 

 not split entirely apart when they divide but remain 

 attached to each other. Soon they become differentiated, 

 that is they grow different in form and function, and 

 finally an animal like the parent is formed. Complete 

 separation, however, occurs in certain worms where an 

 animal may split into two new animals, and also in 

 those organisms which are able to form a new individual 

 from a bud or cut off portion. 



Restricted to Special Cells. — In one-celled organisms 

 any cell may by fission produce a new organism like 

 the parent. In many-celled organisms any cell may 

 divide, but the power to form a new individual like 

 the parent is restricted to certain undifferentiated cells 

 called the reproductive cells. This is necessarily true, 

 for when a cell divides after differentiation the new 

 cell is differentiated in the same way; but if a simple 

 undifferentiated cell divides, the new cells retain the 

 power of differentiation and are able to give rise to the 

 various tissues of the new organism. 



Reproductive Cells of the Sea-Urchin. — Reproduc- 

 tion in a many-celled animal like the sea-urchin is very 



