IO 



THE PHENOMENA OF LIFE 



division. If a cell be mechanically divided, that portion not containing 

 the nucleus dies. 



Histologists have long recognized certain important characters of nuclei. 

 One is their power of resisting the action of various acids and alkalies, particu- 

 larly acetic acid, by which their outlines are more clearly denned, and they 

 are rendered more easily visible. Another is their quality of staining in solu- 

 tions of carmine, hematoxylin, etc. This indicates some chemical difference 

 between the cytoplasm of the cells and their nuclei, as the former is destroyed 

 by these reagents. 



Nuclei are most commonly oval or round and do not necessarily conform 

 to the diverse shapes of the cells; they are altogether less variable elements 

 than cells, even in regard to size, of which fact one may see a good example 



a. 



FIG. 10. Akinesis, Amitosis, or Direct Cell Division. A, Constriction of nucleus; B, 

 division of nucleus and constriction of cell body; C, daughter nuclei still connected by a 

 thread, division being delayed; D, division of cell body nearly complete. (After Arnold.) 



in the uniformity of the nuclei in cells so multiform as those of epithelium. 

 But sometimes nuclei occupy almost the whole of the cell, as in the lymph 

 corpuscles of lymphatic glands, and in some small nerve cells. Their 

 position in the cell is very variable. In many cells, especially where active 

 growth is progressing, two or more nuclei are present. 



Cell Division and Growth. The division of a cell is preceded by 

 division of its nucleus, which may be either direct or indirect. Direct or 

 simple division, amitosis or akinesis, see figure 10, occurs without any change 

 in the arrangement of the intranuclear network. A constriction develops at 

 the center of the nucleus, possibly preceded by division of the nucleoli, and 

 gradually divides it into two equal daughter nuclei. A similar constriction 

 of the protoplasm of the cell occurs between the daughter nuclei and divides 

 it into two parts. 



Indirect division, mitosis, or karyokinesis is the usual method, and con- 



