198 ORIGIN OF SEX AND OF THE SOMA 



development of the soma has been carried much 

 further. Here all the cells of the ccenobium are 

 somatic (Fig. 27), i.e. purely vegetative in function and 

 incapable of division, with the exception of a limited 

 number of purely reproductive or germ cells. These 

 germ cells are of three kinds : (i) so called partheno- 

 gonidia (Fig. 28, A), each of which divides to form a 

 new ccenobium (Fig. 28, B), and thus corresponds in its 

 reproductive function with the ordinary vegetative cells 

 of a Pandorina or of a Eudorina ; (2) gynogonidia 

 (Fig. 28, C), which develop directly into passive (female) 

 gametes (eggs) without flagella, these eggs being very 

 large cells highly stored with food ; and (3) andro- 

 gonidia, each of which divides, as in the case of 

 Eudorina, already described, to form a plate (Fig. 28, D) 

 or a sphere of male gametes (sperms) J which have the 

 same general characters as those of Eudorina. These 

 three types of reproductive cell may be distributed 

 in any combination. Thus a ccenobium may reproduce 

 entirely by means of parthenogonidia, or it may pro- 

 duce eggs only, or colonies of sperms only, or any 

 two, or all three forms of germ cell (Fig. 27). 



There are two species of Volvox found in Britain, V. 

 aureus (Figs. 27, 28, A-D) and V. globator. The former 

 has rounded somatic cells which are separated from one 

 another by considerable spaces, which are bridged bydeli- 



1 Greek airfyfta, seed. This is a useful term applied to all male 

 gametes both in animals and plants ; the male fertilising element has 

 often been called the " seed " in common language. 



FIG. 28. Volvox. A, portion of the surface of a ccenobium showing 

 vegetative cells joined to one another by from one to three proto- 

 plasmic threads : also a parthenogonidium (non-sexual germ 

 cell), x 550. B, partly grown daughter ccenobium derived 

 from the division of a parthenogonidium. x 550. C, egg 

 joined to neighbouring vegetative cells by bundles of protoplasmic 

 threads, x about 700. D, isolated " plate " of male gametes 

 (sperms) derived by division of an androgonidium. x 700. 



