VOLVOX 



305 



colony may divide into 16 zoospore-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. 



d ■^ e 



Fig. 260. — Pandorina moTum. a, Motile colony ordinarily consisting of 

 sixteen motile cells (X 475); 6, colony in which the cells 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 gelatinous 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 



