ONTOGENESIS 



147 



filaments approximate one another, and the contents 

 of the cell of the one strand flow out into those of 

 the other, forming a chain of zygotes (fig. 56), the 

 condition is almost that of isogamy, the only differ- 

 ence between the gametes being that of relative 

 passivity and activity. If the protoplasm of both 

 strands flowed out and fused midway, we would have 

 an example of pure isogamy. Such a condition is 

 found in some species of a relative of Spirogyra, 

 Mougeotia. 



FIG. 58. Eudorina: A, a mature colony (from nature); B, formation 

 of the two kinds of reproductive cells. 



Sexual Differentiation. In Eudorina (fig. 58) 

 we have a colony of sixteen to sixty-four flagellate 

 cells, that, like Stephanosphaera, are imbedded in a 

 gelatinous sphere which they secrete. There are, 

 however, two kinds of these colonies from the 



