18 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 arc 

 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 

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

 often of thousands of ciliated cells (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 arc alike, 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 



