218 



EVOLUTION AND ANIMAL LIFE 



cells of the body: the cells which have to do with the assimila- 

 tion 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 differentiation, 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 re- 



FIG. 124. Spermatozoa of different animals: 1, Man; 2, Vesperugo; 3, pig; 4, rat; 

 5, finch; 6, triton; 7, ray; 8, beetle; 9, mole cricket; 10, snail. (After Ballowitz, 

 Kolliker, and Rath.) 



productive 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, 

 elongate or " tailed " sperm cells. 



In the slightly complex animals one individual produces 

 both egg cells and sperm cells. But in the Siphonophora, or 

 colonial jelly fishes, certain members of the colony produce only 

 sperm cells, and certain other members of the colony produce 

 only egg cells. If the Siphonophora be considered an indi- 

 vidual organism and not a colony composed of many individ- 



