^M THE NEW EVOLUTION '^^^'' 



cell. In other words, on the division of the original 

 cell into two each of these halves may separate from 

 the other and become a separate animal with half the 

 bulk of the original. Further division would giYC 

 rise to a corresponding number of entirely separate 

 animals. The so-called single-celled animals or pro- 

 tozoans (fig. 87, p. 161) illustrate this process. 



But after the division of the original cell into two, 

 four, eight, sixteen, and so on, the cells resulting 

 from division might remain in contact, eventually 

 forming a body consisting of vast numbers of cells. 

 Here there are two alternatives. The cells may 

 adhere more or less irregularly (figs. 1x1-1x5, P- ^^s) 

 so that a poorly differentiated mass of cells results, 

 the mass as a whole being more or less distinctly 

 radial in its symmetry. The result of such develop- 

 ment is represented by the sponges. 



On the other hand, the adhesion of the cells may 

 take place in a regular geometrical fashion (figs. 

 110-117, p. 185) until a hollow ball of cells (called a 

 blastula; figs. 114, 115, p. 185) is formed which, by 

 collapsing like a rubber ball with one side pushed in, 

 would form a two-layered cup (called a gastrula; 

 figs. 116, 117, p. 185) with an axis passing through the 

 center of the opening and of the opposite pole, and 

 the walls the same in all the radii. 



We know that all animals begin life as a single cell 

 which divides into two, and these derivatives con- 

 tinue to divide in the same way. We see that this 

 continued division of the cells takes three different 

 lines. The cells resulting from the divisions may (i) 



[192-] 



