22 



ZOOLOGY 



in function, and varies much in thickness, resistance, etc. 

 Animal cells as a rule are not provided with such well de- 

 veloped and resistant walls as are plant cells. 



39. Cell Functions. Since the cell is only a definite unit 

 mass of protoplasm, its functions are in general those which 

 have already been described as protoplasmic functions. They 



FIG. 6. 



C 



m. 



PIG. 6. Modes of cell reproduction. A, B, and C, stages in the reproduction of the Infusorian, 

 Colpoda, by the breaking up of the protoplasm to form numerous cells. A. encysted stage; B 

 protoplasm escaping, spores partly formed; C, spores completely separated (adapted from Rhum 

 bier); D, budding in Chlamydomyxa, a lowly Rhizopod. The bud is finally constricted off from 

 the mother cell, b, bud; cie. t cell wall; m, mother cell; n, nuclear matter; s, spores. 



Questions on the figure. Compare the process and the results of the two modes 

 of cell reproduction shown in this figure. Can you describe the fate of the 

 "mother" cell in the two cases? 



are merely localized within the cell. The cell wall when present 

 would naturally modify and limit in important ways, the more 

 active protoplasmic functions, especially motion. In such cases 

 the independent motion characteristic of so many cells must be 

 accomplished by special devices. These frequently take the 

 form of cilia or flagella, which are thin protoplasmic projec- 

 tions penetrating the cell wall and used after the manner of 

 oars. Locomotion of cells is not confined to single-celled or- 

 ganisms, but is found in many cells of the higher animals and 

 plants as colorless blood cells, sexual cells, etc., which have 



