REPRODUCTION AND EARLY DEVELOPMENT. 107 



But even when the cells are equal at the start, it soon comes 

 about that the cells in the mass do not have the same 

 relations to the outside world and are thus forced to be 

 different. One such early difference is seen in Fig. 20, 

 A 4, where the cells of one side have been punched into 

 the interior. This condition is very common, and is 

 known as a gastrula. In this there are two layers of 

 cells, one on the outside and the other on the inside, 

 and each comes to have its particular work. 



Often the outside layer develops cilia and thus aids in 

 motion and probably becomes more sensitive. The inside 

 layer lines a cavity which may take in food. Its func- 

 tion then is wholly different : it becomes nutritive. 



From these two layers, by cell division, by growth, 

 and by gradual differentiation of certain regions of cells, 

 we get all the organs of the adult animal of the species 

 to which the egg belongs. It is not the purpose to trace 

 this differentiation in the present book, but it is necessary 

 to say that the organs that come from particular groups 

 of these cells is not a matter of chance or uncertainty. 

 We know not only that a frog's egg will produce a frog 

 and that all frog eggs will develop in the same way, but 

 also that the outside layer (ectoderm, Fig. 20, B, ec) will 

 develop the outer surface of the skin and the nervous 

 system, and that the inner layer (entoderm, Fig. 20, B, 

 en) will finally line the stomach and intestines. 



115. Practically all the many-celled animals start 

 from a single cell and come to be some sort of a rounded 

 mass of cells before they show much differentiation into 

 organs. This means that the complex animals begin 

 life practically as unicellular animals, such as the protozoa ; 

 but it means more than this. It also means that all 



