264 SPERMATOGENESIS AND OOGENESIS LESS, xxm 



this way sixteen daughter colonies are produced within the 

 gelatinous envelope of the original mother colony (B). By 

 the solution of the envelope the daughter colonies are set 

 free, and each begins an independent existence. 



In sexual reproduction the zooids are set free singly from 

 the colony (c). They swim about actively, approach one 

 another in pairs, and conjugate (D), becoming completely 

 fused together (E) to form a zygote (F). This increases in 

 size and develops a thick cell wall (G). After a period of 

 rest, the protoplasm escapes from the cell wall (H), puts out 

 a pair of flagella (i), and swims about. Finally it settles 

 down, divides and re-divides, and so gives rise to a new 

 colony (K). 



It is obvious that Pandorina resembles the polyplast 

 stage of an embryo : moreover it is produced by the re- 

 peated fission of a zygote, just as the polyplast is formed 

 by the repeated fission of an oosperm. 



The beautiful Volvox (Figs 64 and 65), one of the favourite 

 studies of microscopists, is a colony of Haematococcus-like 

 zooids arranged in the form of a hollow sphere containing a 

 transparent mucilage. Each cell (c) has a nucleus, a con- 

 tractile vacuole, a large green chromatophore, a small red 

 pigment-spot like that of Euglena (p. 47) and two flagella. 

 The cells are surrounded by thick mucilaginous cell-walls 

 which do not give the reaction of cellulose, but are probably 

 formed of an allied carbohydrate. By the combined move- 

 ment of all the flagella a rotating movement is given to the 

 entire colony. 



Asexual reproduction takes place by means of certain 

 zooids distinguished from the rest by the absence of flagella, 

 and called parthenogonidia (Fig. 64. A, a). Each partheno- 

 gonidium undergoes a process very like the segmentation of 

 the hydroid egg (p. 246), dividing into 2, 4, 8, 16, &c. cells 



