220 



GENERAL ZOOLOGY 



groups are not distinguishable at all times, as in the case of some 

 of the lower invertebrates. Somatic cells comprise the bulk 

 of the body and include all cells except the reproductive cells, 

 which are the germ cells. In the higher forms these two groups 

 can always be recognized, often from very early stages in develop- 

 ment. In lower forms, Hydra for example, germ cells are indis- 

 tinguishable from somatic cells until the formation of the ovary 

 and testis. 



Somatic Mitosis. — The common method of cell division, 

 known as mitosis, consists of a series of complicated changes 

 which for convenience has been arranged in progressive stages as 

 follows : 



Resting Stage. — When a cell is not dividing, it is said to be in 

 the resting stage, though as a matter of fact it may at the time be 



Fig. 136. — Mitotic figures in cells of larval salamander as they appear in an 

 ordinary section of the skin. X 900. 



actively engaged in the performance of its functional role in the 

 body economy and be resting only in the sense that it is not 

 dividing. Except in the basal layer of the epidermis, somatic 

 mitoses are rare in the adult stage of vertebrates. For the study 

 of somatic cell divisions embryonic stages are preferable since 

 these abound in cell divisions. An embryonic vertebrate cell, 

 say, of the skin or of the liver, consists of a cytoplasmic region in 

 which usually a single nucleus is embedded. Cytoplasm and 

 nucleus are each bounded by a membrane. In the ordinary 

 fixed and stained section, the nucleus contains irregular faintly 

 stained clumps of chromatin and usually one or more rounded 

 bodies called nucleoli. . If the cells are undergoing rapid cell 

 division, a structure known as the centrosome may be seen in the 

 cytoplasm, close to the nuclear membrane. The centrosome is a 

 rounded area composed of compact cytoplasm, in the center of 

 which are two rounded bodies called centrioles. Centrosomes and 



