VOLVOX 305 
colony may divide into 16 zodspore-like cells which remain 
together, escape from the mother colony, and thus become a new 
colony. The gametes are formed in essentially the same way as 
the individuals of the new colonies, but they separate and thus 
swim about independently after leaving the mother colony. 
When the zygospore germinates, as shown in Figure 260, there 
results a new colony which has only to grow to adult size. 
Fic. 260.— Pandorina morum. a, Motile colony ordinarily consisting of 
sixteen motile cells (x 475); 6, colony in which the celis have formed 
daughter colonies (x 475); c, two gametes fusing; d, zygospore; e, zygo- 
spore germinating and forming a new colony. Redrawn with modifications 
from Oersted. 
Among the gametes there is often considerable variation in size and 
motility, some being smaller and more active than others. The 
gametes pair and fuse regardless of their size, and, when gametes 
that are unlike happen to pair, there is a suggestion of heterog- 
amy, although there is no distinct differentiation of gametes as 
occurs in plants where heterogamy is well established. 
Volvox. — The highest expression of colony formation is reached 
in forms like Volvox (Fig. 261), where the colony contains thou- 
sands of individuals held together in a gelatinuus matrix and 
so arranged as to form a hollow sphere. The colonies of Volvox 
are often as large as a pin head and hence visible to the naked eye. 
The two cilia of each individual project from the colony, and by 
the lashing of the cilia the colony moves through the water by a 
revolving motion. One can often see them slowly moving about 
in ditches, ponds, and sometimes in tanks in greenhouses. A 
microscopical study of the colony shows that the individuals of 
the colony are connected by protoplasmic strands, and hence so 
