THE SIMPLEST FORMS OF LIFE 41 



there are not more than a dozen of these large 

 gonidia or reproductive cells. 



Within the group of the Volvocinese there is very 

 perfectly exhibited the evolution of the sexual cells. 

 The lowest members of the series show no marked differ- 

 ences between vegetative and reproductive cells, and 

 the latter are much the same whether they are sexual 

 or non-sexual. Thus, in the genus Pandorina (Fig. 6, 

 F), the sexual cells are hardly distinguishable from the 

 vegetative ones, or those which give rise to a new in- 

 dividual by simple fission ; but these sexual cells sepa- 

 rate, and, escaping from the colony, swim about as 

 unicellular individuals for a short time. Two of these 

 free-swimming cells then come together and fuse into 

 a single one which becomes later a resting-spore, which 

 in time will give rise to new individuals. This fusion 

 of two similar cells is the simplest type of sexual 

 reproduction. 



In the higher Volvocineae, there is a gradual differ- 

 entiation of the reproductive cells, at first indicated by 

 a slight difference in the size of the male and female 

 cells, which are much alike ; but in the genus Volvox, 

 which is the highest of the series, the male cell is very 

 small and ciliated, and is now called a spermatozoid (E), 

 while the female cell is very much larger and quite 

 destitute of motion. This large, non-motile cell is called 

 the egg, or germ-cell (D). 



If we compare the different members of this order 

 we find them forming a continuous series in which dif- 

 ferentiation has proceeded in two directions, while all, 

 nevertheless, retain the primitive power of active loco- 

 motion. While the lower members of the series are 



