THE EVOLUTION OF SEX. 



active and well-nourished cells, a middle layer of heterogeneous con- 

 stituents, can always be distinguished. Every average infusorian is as 

 good as its neighbors, so far as reproduction of new individuals by 

 division is concerned; in the colonial protozoa, the units that are set 

 adrift are very little different from their fellows that remain behind; but 

 this ceases to be true when we pass to colonies where considerable 

 division of labor has been established. It is certainly true that even a 

 tiny fragment of sponge, cut off from the larger mass, may, if it con- 

 tain sufficient samples of the body, and if the conditions be favorable, 

 reproduce a new individual. Cultivaters of bath-sponges habitually 

 take advantage of this fact. But the sponge starts its new colonies for 



Fig. 15. — Volvox, a loose colony of cells, with some set apart for reproduction. — After Kirchner. 



itself usually in quite a different way, namely, by the process of sexual 

 reproduction. Among the cells of the middle stratum of the sponge- 

 body certain well-nourished passive cells appear. These are the ova, 

 at first very like, but eventually well marked from the other 

 constituent units of the layer. Besides these there are other cells, 

 either in the same sponge or in another, which exhibit very different 

 characters. Instead of growing large and rich in reserve material like 

 the egg-cells or ova, they divide repeatedly into clusters of infinitesimal 

 cells, and form in so doing the male elements or spermatozoa. The 

 male and female cells meet one another, they form a fertilized ovum; 

 the result is continued division of the latter till a new sponge is built 

 up. Here, then, there are special reproductive cells, quite distinct 



