224 FOUNDATIONS OF BlOLOGi 



dorsal wall of the coelom, which becomes an integral part of 

 the gonads as development proceeds. 



With regard to the fate of the PRIMORDIAL GERM CELLS, 

 once they have reached testis or ovary, we are on surer ground 

 and can trace with considerable exactness their divisions and 

 transformations which give rise to the gametes, sperm and 

 eggs. In the first place the primordial germ cells proceed to 

 multiply in the testis and ovary so that they produce a large 

 number of relatively small germ cells known as SPERMATO- 

 GONIA and OOGONIA respectively. 



1. Mitosis 



Before taking up the origin of the gametes from the sper- 

 matogonia and oogonia, it will be necessary to describe in 

 some detail the complicated internal process involved in all 

 typical cell divisions, known as MITOSIS, which was dismissed 

 when considering the origin of cells until the reader would be 

 in a position to appreciate to the full its significance. 



Reduced to its simplest terms, a typical resting cell, that 

 is one which is not dividing, consists of a mass of cytoplasm 

 surrounding a nucleus; the latter with its chromatin dis- 

 tributed so that it presents a net-like appearance. In addi- 

 tion to the nucleus, it will be recalled that there is present 

 another important cell organ, the CENTROSOME, which ap- 

 pears like a tiny granule situated in the cytoplasm near the 

 nucleus of the resting cell. For all practical purposes we may 

 consider the cytoplasm as the arena in which mitosis takes 

 place, the centrosome as the dynamic agent, and the nucleus, 

 or more specifically its chromatin, as the essential element 

 which the complicated process is particularly designed to 

 distribute with nicety to the daughter cells which are in pro- 

 cess of formation. With this in mind we may proceed to an 

 outline of the chief stages of mitosis, first cautioning the reader 



