46 Spirazines 



a circumferential groove and will result in the 

 production of daughter cells with rounded ends. 

 However, if the surface structures are the more 

 voluminous, then the interior portions will sepa- 

 rate first and leave the daughter cells with con- 

 cave ends. 



Although a heterogeneous internal structure is 

 always necessary for spontaneous cell division, 

 yet the specific manner in which it is brought 

 about need not always be the same. A single cell 

 may contain as many as three different kinds of 

 self-perpetuating bodies, namely the nuclear 

 structures (chromosomes), the central bodies 

 (centrosomes and asters), and the plastids 

 (leucoplasts and chloroplasts). These three are 

 so different from one another in their structures 

 and behaviors that it would hardly seem possible 

 for all of them to undergo cell division by 

 the same specific method. The nuclear structures 

 and the plastids probably divide as the result of 

 internal stresses which may be produced in vari- 

 ous ways as has already been explained, but 

 division of the central bodies is probably due to 

 entirely different causes. 



A central body consists of a radiating cluster 

 of protoplasmic fibers called the aster, which 

 appears to grow from a tiny central region called 

 the centrosome. Central bodies sometimes ap- 



