76 



ORGANISMS OF TISSUES 



tive or somatic cells, and reproducing or reproductive cells. 

 The differentiation is carried a step farther in the case of Pleo- 

 dorina where twenty-eight of the cells are capable of reproduc-^ 

 ing, while the remaining four cells making up the- thirty-two 

 cell colony are purely vegetative and do not reproduce. Here 

 there is a permanent differentiation in the colony and a long 

 step toward the metazoan condition. In some colonies finally, 

 as in Gonium pectorale the method of development approaches 



FIG. 32. Reproduction of Gonium pectorale. Each of the sixteen cells of the 

 ordinary colony (see Fig. 25) divides until a sixteen-cell stage results; the old 

 colony then breaks up and the sixteen young colonies grow independently. 



closely to that of metazoa. The organism consists of sixteen 

 cells arranged as in Fig. 25. When ready to reproduce, each cell 

 of the colony divides first into two cells; these cells do not 

 separate but while still connected they divide again into four, 

 these four divide into eight and the eight into sixteen. Each 

 cell of the parent organism therefore gives rise to a new colony 

 of sixteen cells, each of which is liberated as a colony by dis- 

 solution of the original jelly mantle (Fig. 32). Here then is 

 a process of development, an embryology in the case of a pro- 

 tozoan, in which all of the constituent cells are potentially 



