COLONIAL PROTOZOA 199 



process. Preceding conjugation the cells of the colony separate 

 and each cell becomes a gamete. These gametes are isogamous, or 

 slightly anisogamous, thus showing the simplest form of sexual 

 differentiation (c/. p. 174). 



In Eudorina elegans, another species of the Volvocidce, the colony 

 consists of eight, sixteen, thirty-two, or even sixty-four flagellated 

 cells resembling those of Pandorina. Daughter colonies are Uke- 

 wise formed by division of the individual cells of the parent colo- 

 nies. Conjugation occurs by union of anisogametes, which are 

 formed from all the cells of the colony as in Pandorina, but in differ- 

 ent colonies, and fuse to form zygotes that produce new colonies by 

 cell division. Eudorina is more complex than Pandorina because 

 the colony is composed of a greater number of cells, and because 

 there are " male " and " female " colonies as well as " male " and 

 " female " gametes. 



Thus in Gonium, Pandorina, and Eudorina, there are gametes; 

 but there is no distinction between body cells and germ cells, 

 because all the cells of these colonies gixe rise to germ cells at the 

 time of conjugation. The cells of such colonies are, therefore, 

 independent in all essential respects and exhibit the condition of 

 physiological balance that characterizes the cells of non-colonial 

 protozoa. In the examples that follow, the distinction may be 

 drawn between body cells, which die a natural death by the 

 disintegration of the colony, and gerrji cells, which are poten- 

 tially immortal since they may unite in fertilization and so con- 

 tinue to future generations. 



Pleodorina illinoisensis consists of a colony of thirty-two cells 

 differentiated into twenty-eight larger cells, which give rise to 

 gametes or germ cells like those of Eudorina, and four smaller 

 somatic or body cells, which are located at one end of the ellipsoidal 

 colony. There is another species, P. californica, in which there 

 are either sixty-four or one hundred and twenty-eight cells, of 

 which approximately one-half are body cells. Hence a division of 

 labor exists among the cells of such a colony, as shown by their 

 structural differences, and by the physiological differences that 

 may be inferred to exist between cells ha\dng such different fates 

 as do gametes and somatic cells. 



In the various species of the genus Volvox, this fundamental 

 di\'ision of labor between somatic and germ cells becomes more 

 conspicuous, since the somatic cells of the Volvox colony greatly 



