42 THE WBENOE AND TEE WHITHEB OF MAN 



responding cells, and thus of rejuvenescence and of 

 giving rise to a new organism. And these cells, form- 

 ing a series through all generations, are evidently im- 

 mortal like the protozoa. Natural death cannot touch 

 them. These are the reproductive cells. The other 

 cells nourish and transport them and carry on the work 

 of excretion and respiration. These latter correspond 

 practically to our whole body. We call them somatic 

 cells. In volvox they are entirely subservient to, and 

 exist for, the reproductive cells, and die when they have 

 completed their service of these. The body is here 

 only a vehicle for ova. Furthermore, in volvox there 

 has arisen such an interdependence of cells that 

 we can no longer speak of it as a colony. The colony 

 has become an individual by division of labor and the 

 resulting differentiation in structure. 



But hydra gives us but a poor idea of the coelente- 

 rata, to which kingdom it belongs. The higher coe- 

 lenterata have nearly or quite all the tissues of higher 

 animals — muscular, connective, glandular, etc. And by 

 tissues we mean groups of cells modified in form and 

 structure for the performance of a special work or 

 function. The protozoa developed the cell for all 

 time to come, the ccelenterata developed the tissues 

 which still compose our bodies. But they had them 

 mainly in a diffuse form. A sort of digestive and re- 

 productive system they did possess. But the work of 

 arranging these tissues and condensing them into com- 

 pact organs was to be done by the next higher group, 

 the worms. 



Let us now take a glance at certain stages of em- 

 bryonic development which correspond to these earli- 

 est ancestral forms. We should expect some such 



