l8 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 
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 are alike, but two kinds of cells may be observed 
in a mature colony: small vegetative ceils which do not divide, and 
among the thousands of these smaller cells a few (rarely over ten or 
