ONTOGENESIS 131 



except such as have arisen from preexisting individ- 

 uals of the same species. 1 



Individuals give rise to other individuals either by 

 simply cutting in two to form two half-organisms, 

 each of which reorganizes its tissue so as to complete 

 itself to the specific type, or by budding off a small 

 portion of itself, which grows and differentiates to a 

 form similar to its parent, or, finally, by budding off 

 a single cell, which grows and differentiates into an 

 individual like its parent. 



Reproduction as a Growth Process. It was 

 pointed out in a previous chapter that, in most cases, 

 cell division is a consequence of 

 cell growth or " cumulative in- 

 tegration " and, in fact, may be 

 considered as a phase of the growth 

 phenomenon, discontinuous instead 

 of continuous. The result is to 

 increase the mass of a tissue with- 

 out differentiation, and without simple budding. 

 the alteration of the size-relations 

 of its components, the cells. In the case of free- 

 living one-celled organisms, cell division results from 

 the same cause, with the difference that the newly 

 formed cell units are free individuals instead of 

 elements of a mass. This is, however, a distinction 

 without any real difference. On the other hand, 

 certain unicellular organisms, after fission, remain 



1 Or in rare cases as mutants from closely related species. See next 

 chapter. 



