40 GENERAL ZOOLOGY 



appears to be the chief bearer of hereditary qualities from 

 one cell to another. How the transfer of hereditary char- 

 acters is believed to take place will be explained later 

 (Chapter XXX). The centrosomes are apparently inac- 

 tive except when the cell divides. They then commonly 

 form dynamic centers which are concerned with the 

 separation of the chromatin material. 



Cells rarely exceed the size which is usual for their par- 

 ticular kind. The largest in animals, such as some nerve 

 cells, are two or three feet long. Most cells, however, are 

 much smaller and usually cannot be seen with the naked 

 eye. The smallest bacteria are less then one twenty-five 

 thousandth of an inch in diameter. When a cell has grown 

 so that it threatens to exceed the maximum set for its kind 

 by natural causes, it divides. Cell-division is important 

 in several ways. It allows cells to grow without becoming 

 too large, and it increases the number of cells. In many 

 celled animals, growth is closely bound up with cell- 

 division and cell-differentiation. Cell-division is, there- 

 fore, a very important biological phenomenon. There are 

 two types: mitotic and amitotic. 



MITOSIS OR INDIRECT CELL-DIVISION 



Mitotic cell-division is a rather complicated process 

 which results in the accurate division of the chromatin of 

 the nucleus and a somewhat less precise separation of the 

 other parts of the cell. The changes which take place are 

 grouped under five stages (shown in Fig. 25) which may be 

 described as follows: 



1. Resting Cell. A resting cell ready for division does 

 not differ from any other cell, except that the nucleus is 

 large in proportion to the cytoplasm. The centrosomes 

 lie at one side of the nucleus and the chromatin is scattered 

 irregularly through the linin network. 



2. Prophase. The centrosomes move apart and take 

 positions on opposite sides of the nucleus. Long strands 



