12 noon 



12-20 p.m 



12-40 p.m. 



12-50 p.m. 



12-53 p.m. 



1-0 p.m. 



Fig. 9 Volvox capensis and V. rousseletii. A-J, stages in the inver- 

 sion of a daughter colony. A, denting begins. C, dents smooth out. 

 D, colony round again. E, 'hour-glass' stage. F, posterior half 

 contracts. G, infolding begins. H, infolding complete. I, posterior 

 half emerges through phialopore. J, flask stage begins. K, flask 

 stage ends. L, inversion complete. (All x 150 approx.) (After 



Pocock.) 



gested that the volvocine colony has arisen at least twice in the 



course of evolution, once from a Sphaerella and once from a 



Chlamydomonas ancestry. Some evidence in support of this view 



has recently come to hand from a study of chromosome numbers 



(see below). On the other hand, the great uniformity of their sexual 



reproduction can be employed as an argument against such a di- 



phyletic origin. 



30 



