128 PROTOZOA 



CHAP. 



the case may be. These vegetative cells have therefore surren- 

 dered the power of fission elsewhere inherent in the Protist cell. 

 Moreover, when the sphere ruptures for the liberation of the 

 young colonies, it sinks and is doomed to death, whether because 

 its light-loving cells are submerged in the ooze of the bottom, 

 or because they have no further capacity for life. When conju- 

 gation is about to take place, it is the cells that otherwise would 

 be parthenogonidia that either act as oospheres or divide as 

 " spermogonia " to form a flat brood of minute yellow male cells 

 (" sperms "). These resemble vegetative cells, in the possession 

 of an eye-spot and two contractile vacuoles, but differ in the 

 enormously enlarged nucleus which determines a beaked process 

 in front. After one of these has fused with the female cell 

 (" oosphere ") the product (" oosperm ") encysts, passes into a 

 stage of profound rest, and finally gives rise to a new colony. 

 The oospheres and sperm-broods may arise in the same colony or 

 in distinct ones, according to the species. 



Before we consider the bearings of the syngamic processes of 

 VolvoXy we will study those presented by its nearer allies, which 

 have the same habitat, but are much more minute. Three of 

 these are well known, Stephanosphaera, Pandorina, and Eudorina, 

 all of which have spherical colonies of from eight to thirty-two 

 cells embedded at the surface of a sphere, and no differentiation 

 into vegetative cells and parthenogonidia (or reproductive cells). 



Stephanosphaera has its eight cells spindle shaped, and lying 

 along equidistant meridians of its sphere ; in vegetative repro- 

 duction each of these breaks up in its place to form a young colony, 

 and the eight daughter-colonies are then freed. In conjugation, 

 each cell of the colony breaks up into broods of 4, 8, 16, or 32 

 small gametes, which swim about within the general envelope, and 

 pair and fuse two and two : this is " isogamous," " endogamous " 

 conjugation. In Pandorina (Fig. 45) the cells are rounded, and are 

 from 16 to 32 in each colony. The vegetative reproduction in 

 this, as in Eudorina, is essentially the same as in Stephanosphaera. 

 In conjugation the cells are set free, and are of three sizes in 

 different colonies, small (S), medium (M), and large (L). The 

 following fusions may occur : S x S, S x M, S x L, M x M, M x L. 

 Thus the large are always female, as it were, the medium may 

 play the part of male to the large, female to the small ; the 

 small are males to the medium and to the large. The medium 



