GENERATIOX, SEX AM J UXTOGEX\ 217 



its nucleus. Then the two individuals separate and cacli 

 divides into two. I'he result of this conjugation is to give to 

 the new P«ra???ffrm- produced by the conjugating individuals 

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

 individuals. The new Paramacia arc not simply halves of a 

 single parent, they are parts of two parents. 



Among tlic colonic! Protozoa the first differentiation of 

 the cells or mom])ers comi)osing the colony is the difTerentia- 

 tion into two kinds of iei)roductive cells. Reproduction by 

 cimple division, without i)receding conjugation, c^n and does 

 take i)lace, to a certain extent, among all the coloniid Protozoa. 

 Indeed, this simple method of multii)li('a(ion, or some modi- 

 fication of it, like l)udding, persists auKjng many of the com- 

 plex animals, as the sponges, the polyps, r.nd 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 j)Ower of nuiltiplica- 

 tion in this way always exhibit also the other more comj)lex 

 kind of multiplication, the method of doul)le-])arent re])ro(luc- 

 tion. Conjugation takes place between different meml)ers 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 Pandoriim they apjiear to be slightly 

 different, and in Eudorina and ^^olvox 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, 

 wliile the other kind, the sperm cell, is small, with (n-oid head 

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

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

 but in ]^olvox only certain cells i)erform this function, and the 

 other cells of the bodv die. Or we may sav that the bodv of 

 Volvox dies after it has ])roduced sjiecial reproductive cells 

 which shall fulfill the function of multii)lication. 



Beginning with the more comi)lex ]'olvocinu\ whicli we 

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

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

 animals show this distinct differentiation between the rei>ra- 

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

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

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



