MITOSIS 49 



These sections are then treated with stains or dyes. Various parts 

 within the cell react differently to the stain and for that reason 

 are easily distinguishable. Chrowatin, one of the materials within 

 the nucleus, is especially deeply colored by the stains most fre- 

 quently used. 



In interpreting sections of cells in this study keep the follow- 

 ing facts in mind: 



1. The nucleus is much smaller than the entire cell, conse- 

 quently many slices through a given cell will contain no part of 

 the nucleus. 



2. Before mitosis has begun the nucleus is very conspicu- 

 ous as a large light colored body in which some darker granules 

 of chromatin are found. 



3. Early in the process of mitosis the membrane surround- 

 ing the nucleus disappears, allowing the nuclear material to lie 

 directly in the protoplasm. From this time there is no sharply 

 defined light colored body to represent the nucleus. 



4. The spindle formed by the centrosome has but one chief 

 axis. The developing eggs are too small to be placed all in a 

 uniform position, consequently when sections are cut only part 

 of them will pass through the spindle and of these only a very 

 small percentage will contain the entire spindle. Most of the 

 sections passing through the spindle will cut it at various angles 

 to its chief axis and will thus include only a portion of the mi- 

 totic figure. 



In sections of the eggs and embryos of Cerebratulus or in 

 sections of the spermary of the Salamander study the following 

 stages in the process of mitosis and make one or more drawings 

 of each. Divide the page into six parts and plafe drawings in 

 proper order as the different stages are found. 



For the resting stage draw the entire cell. In the remaining 

 stages draw the nuclear structures and mitotic figure only. 



