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 Volvocinex 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 Volvocinex, 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 
