392 THE EVOLUTION OF MAN. 



Hydroid Polyps. In the Haliphysema (Fig. 315) and in 

 the Olynthus the whole body is a simple intestinal pouch, 

 which is only essentially distinguished from the gastrula by 

 the fact that it is adherent by the end opposite the mouth. 

 The thin wall of the pouch consists only of the two 

 primaiy germ-layers. As soon as it is sexually mature, 

 single cells of the wall become female egg-cells, others 

 become male sperm-cells, or seed-cells; the former grow 

 very large, as they form a considerable number of yelk- 

 gTanules in their protoplasm (Fig. 181, e); the latter, on the 

 contrary, by continued division, become very small, and 

 modify into movable "pin-shaped" spermatozoa (Fig. 17, 

 vol. i. p. 173). Both kinds of cells sever themselves from their 

 birthplace, the primary germ-layers, fall either into the 

 surrounding water or into the intestinal cavity, and there 

 unite by amalgamation. This is the very important process 

 of the fertihzation of the egg-cell by the sperm-cell. (Cf 

 Fig. 18, vol. i. p. 175.) 



These simplest processes of sexual reproduction, as 

 exhibited at the present time in the lowest Plant Animals, 

 especially in the Chalk Sponges and Hydroid Polyps, inform 

 us of several extremely important and significant facts ; 

 in the first place, we learn, that for sexual reproduction in 

 its simplest form, nothing more is required than the 

 blending or amalgamation of two diflTering cells, a female 

 egg-cell and a male sperm-cell, or seed-cell. All other 

 circumstances, and all the other extremely complex pheno- 

 mena, accompanying the act of sexual reproduction in the 

 higher animals, are of a subordinate and secondary charac- 

 ter, and have only attached themselves secondarily to that 

 simplest primary process of copulation or fertilization, or 



