172 ESSENTIALS OF BIOLOGY 



of cells becomes sufficient for the body, or bodily part, or tissue 

 that is growing. 



a. The finer aspects : Mitosis. The cell is a complex of 

 parts. 



It is convenient to orientate it so as to indicate " equatorial " 

 and " polar " regions. In it there is a rounded body — the nucleus, 

 which is bounded by a nuclear membrane. The substance of the 



FoLe 



UcfizoutorijaZ 

 plane 



^ qraatorLod 



Cell merribroine 

 ^NujcLeoLv membranje 

 .,- ChromosoTnes 



■ CytoplcLsm 



pLojm 



Fig. 22. — Essential Structures in a Cell about to Divide. 



1-6, Places in the mitotic division of a cell. 



cell peripheral to the nucleus is called cytoplasm and in the latter 

 there are bodies (" mitochondria," " Golgi-bodies," " centro- 

 somes," etc.) which do not concern us here. Within the nucleus 

 the most significant substance is the chromatin (see Section 73^). 

 In general the chromatin does not appear to have any very definite 

 arrangement in the nucleus unless the cell is about to divide. 

 Then it becomes assembled as a series of little rods, the chromo- 

 somes, and the number and general arrangements of these are very 

 characteristic. In all the cells of the tissues of an animal or plant 



