196 



ORIGIN OF SEX AND OF THE SOMA 



body which takes no part in reproduction. It is 

 difficult to say why these particular cells should be 

 incapable of division. Perhaps it may be connected 

 with a more highly developed function for perceiving 

 the direction of light. It is generally true that speciali- 

 sation of vegetative function in a cell carries with 

 it loss of reproductive power. It is noteworthy that 

 all transitions are found between the Eudorina condi- 

 tion, in which all the cells are equal and capable of 



A 



FIG. 26. Pleodorina. A, P. illinoiensis. Ccenobium consisting of 

 28 large cells capable of division to form new coenobia and 4 small 

 cells at the front end of the colony which do not divide and die 

 when the new ccenobia formed by the large cells are liberated. 

 B, P. californica. Sterile (somatic) cells more numerous. The 

 arrow indicates the direction of locomotion in both cases. 



division, and the occurrence of these sterile cells, which 

 may vary both in size and in number from one to 

 twelve, and that the capacity for division depends upon 

 the size of the cell. Pleodorina illinoiensis is in fact con- 

 sidered to be a state of Eudorina, and thus we have 

 the appearance of the soma, a fundamentally im- 

 portant step in evolution, first arising as a fluctuating 

 condition within the limits of a species. 



A larger form of Pleodorina (P. californica) has 



