10 



ANIMAL BIOLOGY 



essentially naked masses of protoplasm, such as Amoeba and white 

 blood cells. In other words, the protoplasm is the actual living 

 part — the cell wall typically being a non-living accessory which 

 more or less sharply separates one unit mass of protoplasm from 

 another and lends rigidity and form to the group of cells as a whole. 

 The living material of cells is highly organized into various 

 complex structures, some of which are present in all cells and 

 others only in cells adapted for special functions. For the present, 



Gastric vacuole 



Nucleus 







r V: v. . 





n 



Endoplasm 

 Contractile vacuole Ectoplasm 



Fig. 6. — A simple animal (Amoeba proteus) which consists of a single cell 

 (highly magnified). Locomotion is by streaming of the protoplasm forming 

 temporary protrusions, or pseudopodia. 



however, it is only necessary to emphasize that the protoplasm of 

 all typical cells is differentiated into two chief parts: the cyto- 

 plasm, or general groundwork which makes up the bulk of the cell ; 

 and the nucleus, a more or less clearly defined spherical body, 

 situated near the center of the cytoplasmic mass. 



Cytoplasm and nucleus, looked at from the functional view- 

 point, represent a physiological division of labor within the con- 

 fines of the cell. Experiments have shown that they are mutually 

 necessary for cell life; the removal of the nucleus putting an end to 

 constructive processes — assimilation, repair, and growth — and 

 thus leading rapidly to death. Accordingly the nucleus may be 

 considered as the center of the synthetic activities of the cell, and 

 the cytoplasm, if not as the area in which destructive processes 

 are chiefly involved, at least as the region in which the results of 

 the nuclear activity become realized. It seems clear that the 

 nucleus is the controlling agent in cell activity, and hence a 



