i8 



MORPHOLOGY 



process of fusion in this case is called fertilization, and the product is an 

 oospore (fertilized egg). 



It is evident that in passing from isogamy to heterogamy there is a 

 differentiation of sex, so that we recognize a male gamete and a female 



FIG. 28. Eudorina: cells of the colony functioning as eggs, with which sperms are 

 coming into contact ; above the colony a group of sperms still hanging together may be 

 seen. After GOEBEL. 



gamete. The female gamete has developed its nutritive supply, and 

 hence its size, at the expense of activity, and finally becomes an entirely 

 passive cell; while the male gamete retains its activity. 



Volvox. In this form the highest expression of colony formation is 

 reyched, the free-swimming colony being a hollow sphere composed 

 often of thousands of ciliated cell^ (figs. 29, 30). These cells are con- 

 nected by strands of cytoplasm, and therefore the structure may be 

 regarded as a multicellular individual rather than as a colony. At first 

 all cells of the colony are alik^, but two kinds of cells may be observed 

 in a mature colony: small vegetative cells which do not divide, and 

 among the thousands of these smaller cells a few (rarely over ten or 



