134 



CELLS OF THE VERTEBRATE BODY 



the definition is to hold for all cases, it must be modified to the 

 effect that a cell is a mass of protoplasm which, at some stage in its 

 development, contains a nucleus. This statement, together with 

 the definition of a cell as the unit of structure and function, is of 

 fundamental importance in further consideration of cells. Struc- 

 turally, cells are divided into two main parts, the cytosome, or cell 

 body, and the nucleus. In the following account these parts will 



cm. 



cso 



n.m 



nu 



f - 



» 



"*< 







y 



^i 





v- 



T ^ 



ah V:r lA^ 



m r, 



V - 



^^'TS nz\-^)t — n.h. 



c.r. 



Fig. 74. — Diagram of a typical animal cell. 



cm., cell membrane; c.r., cytoplasmic reticulum; cso, centriole or centrosome; csp, 

 centrosphere; /, fat; y.b., Gol;?! elements; I, linin; m, mitochondria; n.k., net knot of 

 chromatin granules; n.m., nuclear membrane; n«, nucleolus; )i, vesicles; ^, yolk. 



be considered as they occur in what may be called a typical cell 

 (Fig. 74). 



The Cytosome. — The cytosome is limited externally Ijy the 

 cell membrane or plasma membrane, which is generally regarded 

 as a firmer layer of the cytoplasm. In some cases a reticula- 

 tion is indicated in the cytosome, but this is not a constant 

 feature. Lying near the nuclear membrane, a differentiated, 



