GREAT STEPS IN EVOLUTION 89 



^sidering the simplest expressions of the sex- 

 difference, as we see it, for instance, in 

 Volvox, an interesting colonial Infusorian, 

 which well illustrates a body in the making. 

 It is a beautiful rolling ball of ciliated cells, 

 and these component units are connected 

 by protoplasmic bridges. From the ball of 

 cells reproductive units are sometimes set 

 adrift, which divide to form other colonies 

 without more ado. But in other conditions, 

 when nutrition is checked, a less direct mood 

 of reproduction occurs. Some of the cells 

 in the ball become large, well-fed elements 

 ■ — the ova; others, less anabolic, fade from 

 green to yellow, divide and re-divide into 

 many minute units — the spermatozoa. The 

 large cells of one colony are fertilized by 

 the small cells from another. Here we see 

 the formation of dimorphic reproductive 

 cells in different parts of the same organism. 

 But we may also find Volvox balls in which 

 only ova are produced, and others in which 

 only sperms are produced. The former seem 

 to be more vegetative and nutritive than 

 the latter; we call them female and male 

 organisms respectively; we are at the founda- 

 tion of the differences between the two sexes. 

 Again we would state our thesis that all 

 through the animal series, from active Infu- 

 sorians and passive Gregarines, to feverish 



