GENERATION, SEX AND ONTOGENY 217 



its nucleus. Then the two individuals separate and each 

 divides into two. The result of this conjugation is to give to 

 the new Param&cia produced by the conjugating individuals 

 a body which contains part of the body substance of two distinct 

 individuals. The new Paramoecia are not simply halves of a 

 single parent, they are parts of two parents. 



Among the colonial Protozoa the first differentiation of 

 the cells or members composing the colony is the differentia- 

 tion into two kinds of reproductive cells. Reproduction by 

 simple division, without preceding conjugation, can and does 

 take place, to a certain extent, among all the colonial Protozoa. 

 Indeed, this simple method of multiplication, or some modi- 

 fication of it, like budding, persists among many of the com~ 

 plex animals, as the sponges, the polyps, and even higher and 

 more complex forms. But such a method of single-parent 

 reproduction cannot be used alone by a species for many gener- 

 ations, and those animals which possess the power of multiplica- 

 tion in this way always exhibit also the other more complex 

 kind of multiplication, the method of double-parent reproduc- 

 tion. Conjugation takes place between different members of 

 a single colony of one of the colonial Protozoa, or between 

 members of different colonies of the same species. These 

 conjugating individuals in the simpler kinds of colonies, like 

 Gonium, are similar; in Pandorina they appear to be slightly 

 different, and in Eudorina and Volvox the conjugating cells are 

 readily seen to be very different from each other. One kind of 

 cell, which is called the egg cell, is large, spherical, and inactive, 

 while the other kind, the sperm cell, is small, with ovoid head 

 and tapering tail, and free-swimming. In the simpler colo- 

 nial Protozoa all the cells of the body take part in reproduction, 

 but in Volvox only certain cells perform this function, and the 

 other cells of the body die. Or we may say that the body of 

 Volvox dies after it has produced special reproductive cells 

 which shall fulfill the function of multiplication. 



Beginning with the more complex Volvocince, which we 

 may call either the most complex of the one-celled animals or 

 the simplest of the many-celled animals, all the complex 

 animals show this distinct differentiation between the repro- 

 ductive cells and the cells of the rest of the body. Of course, 

 we find, as soon as we go up at all far in the scale of the animal 

 world, that there is a great deal of differentiation among the 



