12 



ANIMAL BIOLOGY 



primary factor in growth, development, and transmission of specific 

 qualities from cell to cell, and so from one generation to the next. 



B. Cell Division 



All the evidence indicates that, at the present time at least, 

 living matter never arises except under the influence of preexist- 

 ing living matter. That is, protoplasm grows — cells grow and, 

 having attained a certain size, reproduce by dividing into two es- 

 sentially equal parts. Then there are two cells — the parent cell 



Fig. 8. — An Amoeba in six successive stages of division. The dark body 

 surrounded by a clear area is the nucleus. (Modified, after Schultze.) 



has lost its identity in its offspring. Cell division is reproduction. 

 Indeed, in final analysis reproduction is always cell division, 

 through this primary fact is largely obscured by accompanying 

 phenomena in higher animals and plants. (Fig. 8.) 



The process of cell division involves the division of both cyto- 

 plasm and nucleus, and therefore we must enlarge our concep- 

 tion of a cell as a small mass of protoplasm differentiated into 

 cytoplasm and nucleus, by adding that both cytoplasm and 

 nucleus arise through the division of the corresponding elements 

 of a preexisting cell. 



