216 REPRODUCTION 



and physiologically independent. In Pandorina {cf. p. 198), each 

 of the sixteen cells divides into sixteen, forming daughter colonies, 

 which separate, grow to full-sized colonies, and repeat the process. 

 These processes are obviously comparable with similar ones in the 

 preceding types. The only difference is the temporary association 

 of the cells to form the colonies. At the time of sexual reproduc- 

 tion the cells separate and become gametes, which conjugate to 

 form zygotes comparable with those of the non-colonial types. 

 In Type 4 there is, therefore, a period in which the cells are asso- 

 ciated in colonies and another in which the species is represented 

 by isolated cells from which colonies again develop by cell division. 

 Th3 gametes cf such a species may be isogamous or anisogamous. 

 In Pandorina they are usually isogamous, but they may be slightly 

 anisogamous {cf. p. 199), in which case they show the beginnings 

 of sexual diiferentiation. In the related genus Eudorina, there 

 are more cells in the colony and the gametes are markedly 

 anisogamous. Moreover, the male and female gametes of Eudor- 

 ina are produced in different colonies. We might include in 

 the table a Type 4a which would be more speciahzed after the man- 

 ner of Eudorina. 



Type 5 represents the cycle in organisms like Pleodorina and 

 Volvox (Fig. 106, p. 200), in which colonies arise by division of 

 single cells as in Pandorina, but in which the cells are differenti- 

 ated as body or somatic cells and gametes or germ cells. The 

 unbalanced physiological state of the cell that thus arises has been 

 mentioned in the special account of these colonial Mastigophora 

 (p. 201). From the standpoint of reproduction, the important 

 difference between these two kinds of cells in Type 5 of the table 

 is that the body cells die a natural death with the disintegration of the 

 colony, while the germ cells are immortal in the sense that they may 

 live if they unite in conjugation. In the preceding types the cells 

 may die, as most of them do, by the accidents of nature ; but there 

 is no death that is " natural " in the sense that it is inevitable for 

 some of the cells at one phase of the Ufe cycle. In Type 5 the 

 body cells must perish while the gametes alone are immortal like all 

 the cells in the preceding types. In this respect a form like Volvox 

 can perhaps be called a many-celled rather than a single-celled 

 organism, since the differentiation between body cells and germ 

 cells is the most distinctive step in the series of cell cycles repre- 

 sented by the table. 



