56 ANIMAL LIFE 



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 Eudo- 

 rina and Volvox the conjugating cells are very different from 

 each other (Figs. 15 and 16). 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 repro- 

 duction, but in Volvox only certain cells perform this func- 

 tion, 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 multi- 

 plication. 



Beginning with the more complex Volvocinse, 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 re- 

 productive 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 differentia- 

 tion among the cells of the body : the cells which have to 

 do with the assimilation of food are of one kind ; those on 

 which depend the motions of the body are of another kind ; 

 those which take oxygen and those which excrete waste 

 matter are of other kinds. But the first of this cell differ- 

 entiation, as we have already often repeated, is that shown 

 by the reproductive cells ; and with the very first of this 

 differentiation between reproductive cells and the other 

 body cells appears a differentiation of the reproductive 

 cells into two kinds. These two kinds, among all animals, 

 are always essentially similar to the two kinds shown by 

 Volvox and the simplest of the many-celled animals namely, 

 large, inactive, spherical egg cells, and small, active, elon- 

 gate or " tailed " sperm cells. 



