3o6 



ELEMENTS OF BIOLOGY 



cells restores a greater area in proportion to volume, but this is 

 certainly not the whole cause of cell division. Nor are the physical 

 and chemical processes that cause the nuclear phenomena about to 

 be described, known. We are limited therefore to descriptions of 

 the process as seen under the microscope. 



Amitosis and Mitosis. In the final division of cells in the dif- 

 ferentiation of some tissues during development, and under cer- 

 tain other conditions, the nucleus divides by separating into unequal 

 fragments which then become the nuclei of daughter cells. This 



form of cell division is known as 

 amitosis; it is said that a cell having 

 once divided by amitosis does not 

 again divide in any other way. Its 

 occurrence is, to say the least, infre- 

 quent and in all observed cell divisions 

 concerned with the establishment of 

 the individual, cell nuclei do not divide 

 in this irregular and simple fashion. 

 The usual division process is much 

 more complicated. The indirect type 

 of division, involving a regular and 

 equational division of nuclear mate- 

 rials, is known as mitosis. Since inheritance in the last analysis 

 depends on this type of cell reproduction, it is necessary to become 

 familiar with the details of the process. 



Chromatin. In the nucleus of all cells may be found a number 

 of different materials; chromatin (Fig. 196) is a type of substance 

 that is common to all. Study of the mechanism of inheritance has 

 closed in on chromatin as the material which plays an all important 

 part and is apparently the container of inheritable qualities that are 

 transmitted from generation to generation. As yet the chemical prop- 

 erty of the chromatin that is responsible for its extraordinary powers 

 is unknown; nothing that is very significant is revealed by the 



Fig. 196. — A cell; nucleus not 

 undergoing any phase of cell divi- 

 sion. Note the distribution of the 

 chromatin. 



