112 GENERAL BIOLOGY 



as in Closterium. Spiroyra shows the slight difference 

 that one of its conjugating cells is slightly more active, its 

 protoplasm passing over bodily into the other 

 one. In Epistylis (fig. 54) there is greater difference in 

 size and the active cell possesseslocomotory apparatus and 

 is free-swimming. 



It may occur between cells of reduced size, such as the 

 swarm spores of the Sporozoa, next to be studied (fig. 68). 

 But it usually occurs, even among the simpler organisms, 

 between two specialized reproductive cells (gametes) of 

 opposite character: 



SPERM small — active — with little cytoplasm 

 (or spermatozoan) and no inclusions. 



EGG large — -receptive — cytoplasm charged with 

 (or ovunt) food materials. 



This alone is sexual reproduction, since in absence of this 

 differentiation there is no sex. Sex cells are well illustrated 

 in Volvox. The many generations of cells produced by 

 fission remain in contact and constitute the spherical colony. 

 A few scattered cells in the colony cease their vegetative 

 activity and become differentiated in two ways. Some of 

 them repeatedly divide until very small, develop swimming 

 flagella, and are liberated as free-swimming sperms. The 

 others increase in size and food content by storing up in 

 their cytoplasm food materials derived from neighboring 

 cells, and become eggs. At maturity the sperms break out 

 and swim abroad. The egg remains where produced, and 

 develops only if some wandering sperm find it and fuse 

 with it. After such fusion (fertilization) , ordinary cell divi- 

 sion begins again; and the little cluster of cells soon formed 

 is set free as a new minute volvox colony. 



The case of Volvox is especially significant because Volvox 

 is like all the higher organisms in the possession of true sex 



