FLAGELLATA 



127 



plate bends up (as in the gastrulation of the double cellular plate 

 of the Nematode CucuUanus, Yol. II. p. 136), and finally forms a 

 hollow sphere bounded by a single layer of cells : the site of the 

 original orifice may be traced even in the adult as a blank space 

 larger than exists elsewhere. Among the cells of the young 

 colony some cease to divide, but continue to grow at an early 

 period, and these are destined to become in turn the mothers 



.^ ofy 



Fio. a.— Volvox globator. A, entire colony, enclosing several daiigliter-colouies ; B, 

 tlie same during sexual maturity ; C, four zooids in optical section ; D'-D", develop- 

 ment of parthenogouidium ; E, ripe spermogonium ; F, sperm ; G, ovum : H, 

 oosperm, a, Partlienogonidia ; fl, fiagellum ; m, ovum ; my, ovaries ; pg, pigment 

 spot ; sp, sperms ; Spy, spermogonia dividing to form sperms. (From Parker s 

 Biology, after Cohu and Kirctner.) 



(" parthenogonidia ") of a new colony; they begin segmenting 

 before the colony of which they are cells is freed. The young 

 colonies are ultimately liberated by the rupture of the sphere as 

 small-sized spheres, which henceforth only grow by enlargement 

 of the sphere as a whole, and the wider separation of the vege- 

 tative cells. Thus the vegetative cells soon cease to grow ; all the 

 supply of food material due to their living activities goes to the 

 nourishment of the parthenogonidia, or the young colonies, as 



