192 ORIGIN OF SEX AND OF THE SOMA 



outside, just as in the case of the walled isogametes 

 in Fig. 22. 



In forms which show a more complete differentiation 

 of sex, as we shall presently see, not only the size 

 but also the structure of the two conjugating gametes 

 is very markedly different, so that the large one 

 (female) is necessarily quite passive, and the small 

 (male) is alone active in the process. 



We now have to consider a series of forms which 

 do not live singly like Ohlamydomonas, but in which 

 the cells produced by ordinary division, each of which 



Fig. 24.— Unusual type of conjugation in the same species. The 

 gametes have met obliquely, and the protoplasm of both has 

 emerged from the cell walls to form a spherical zygote outside. 



has the same essential structure as a Ghlamydomonas 

 cell, remain together, surrounded by a common (muci- 

 laginous) envelope, through which the flagella pro- 

 trude. The colony or coenobium ^ of cells so formed 

 behaves like a single organism, moving through the 

 water by the co-ordinated beating of the flagella of 

 all the cells. The first form we shall consider is 



Pandoriua, which is a spherical coenobium of (usually) 

 16 cells pressed closely together, so that each cell is 

 somewhat wedge-shaped (Fig. 25, A). In reproduction 

 each cell of the coenobiurii divides by 4 closely follow- 

 ing bipartitions to produce 16 cells, and each group 

 of 16 cells forms a new coenobium (Fig. 25, B), the 



' Greek koiv6s, common, and pCog, life. 



