76 THE LIVING WORLD. 



Since they were protected from internal injury, they 

 could, on the other hand, better perform certain 

 functions connected with the preparing of the food, 

 provided that they could succeed in getting hold of 

 the food itself. The outer layer of cells, coming into 

 direct contact with the world, retained all of the 

 powers which enabled it to be stimulated by that 

 world, and soon learned to pass food to the inner 

 layer of cells for digestion. 



We are still in the dark as to the exact manner in 

 which the special form known as the Gastraea arose. 

 According to some embryologists, the cells first 

 formed a hollow sphere, and then one side of it was 

 infolded, as one would push in the side of a hollow 

 rubber ball. According to others, the mass of cells 

 was solid, the outer ones soon becoming different 

 from the inner ones, and later a cavity appeared in 

 the middle, which broke through to the exterior at 

 one end, and this opening formed the mouth. Ac- 

 cording to still another view, the cells at first formed 

 a flat mass of two layers of cells, and this by folding 

 up into a cup, or going through other modifications, 

 became the two-layered sac which forms the first 

 distinct stage in the development of the multicellular 

 animals. According to others still, and this is the 

 most recent view, a hollow sphere was formed, and 

 thus cells from the shell migrated into the interior, 

 one by one, to form the internal layer. The mouth 

 opening was developed later. But whatever differ- 

 ence there may be in our ideas as to the details of 

 this formation, no biologist questions the fact that 

 very early in the history of the living v/orld there 



