VERTEBRATES: BACKBONE AND BRAIN 101 



The first function to have cells specially devoted to it 

 is the reproductive ; this is a function absolutely neces- 

 sary for the maintenance of the species. For the nu- 

 tritive cells die when they have brought the reproduc- 

 tive cells to their full development. These few nutri- 

 tive cells represent the body of all higher animals in 

 contrast with the reproductive elements. And with 

 the development of a body, death, as a normal process, 

 enters the world. The dominant function is here evi- 

 dently the reproductive, and the whole body is sub- 

 servient to this. 



In hydra the union and differentiation of cells is 

 carried further. But the cells are still much alike and 

 only slowly lose their own individuality in that of the 

 whole animal. This is shown in the fact that each 

 entodermal cell digests its own particles of food, 

 although the nutriment once digested diffuses to all 

 parts of the body. Also almost any part of the ani- 

 mal containing both ectoderm and entoderm can be 

 cut off and will develop into a new animal. 



But beside the reproductive cells and tissues hydra 

 has developed a very simple digestive system, in which 

 the newly caught food at least macerates and begins 

 to be dissolved. This is the second essential function. 

 The animal can, and the plant as a rule does, exist 

 with only the lowest rudiments of anything like ner- 

 vous or muscular power ; but no species can exist 

 without good powers of digestion and reproduction. 

 These essential organs must first develop and the 

 higher must wait. And the inner, digestive, layer of 

 cells persists in our bodies as the lining of the mid- 

 intestine. We compared hydra therefore to a little 

 patch of the lining of our intestine covered with a flake 



