ORGANIZATIONAL UNITS 29 



a certain size, reproduce by dividing into two more or less 

 equal parts. The process of cell division involves the divi- 

 sion of both cytoplasm and nucleus, and therefore we must 

 enlarge our conception 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. 



We shall later have occasion to make a study of the details 

 of cell division, known as MITOSIS, but from what has been 

 said it must occur to the reader that, since cells arise only 

 by division, those of the present day, whether complete 

 free living organisms or units composing the bodies of higher 

 animals and plants, are actually lineal descendants in un- 

 broken series from the beginning of life on the Earth. The 

 bond of discontinuity between parent and offspring is typi- 

 cally a single cell division, (Fig. 123.) 



