THALLOPHYTES 



19 



t welve) much larger on es 

 which divide to form ne w 

 colonies. These large 

 colony-forming cells are 

 derived from the smaller 

 cells and have been called 

 gonidia, a very inappropri- 

 ate name. 



The sexual reproduction 

 is much as in Eudorina, 

 but the eggs become much 

 l arger than^ thp orHinary 

 cells and lose their ci lia. 

 The sperms, produced by 

 the division of certain 

 cells, are elongated, yellow, 

 and biciliate. Fertihzation 



Fig. 29. — Volvox: the large globular colony 

 composed of small vegetative cells connected by 

 strands of cytoplasm, two large colony-forming cells, 



sLiitiiub 01 cyLupiastii, Lwo large coiony-ioi 

 occurs in the cavity of the and numerous oospores with rough walls." 



colony (fig. 31), and the 

 resulting oospore is a resting, protected cell (fig. 32). Upon germina- 

 tion the oospore produces a swarm of active cells (zoospores) that 

 cling together and organize a new colony. 



In this so-called Volvox colony differentiation has resulted in four 



distinct kinds of cells: ciliated vegeta- 

 tive cells, colony-forming cells, eggs, 

 and sperms. 



Conclusions. — A summary of the 

 features of the Volvocales may be 

 stated as follows : The forms range 

 from isolated cells to complex spheri- 

 cal colonies, all the ordinary cells 

 being cihate; a new colony is formed 

 from the division of a single mother 

 cell; sexual reproduction is present, 

 advancing from isogamy to heter- 

 ogamy, that is, from the origin of sex 

 Figs. 30-32. — Fafooa: ; 30, periph- jq t]^g differentiation of sex. Volvox 

 eral cells of the colony fafter West); , . . ... 



3 1, an egg surrounded by sperms; 32, an ^nd its Colony-forming allies are tO 



oospore with heavy wall. be regarded as specialized forms, and 



