26 THE GROWTH OF THE BRAIN. 



an emphasis of one of the several powers originally 

 common to them all. The specialised cell is therefore 

 physiologically unbalanced, if not incomplete; and so the 

 more exquisitely a cell is adapted to some particular 

 function, the less capable it is of performing the .entire 

 series of reactions indispensable to its very existence ; 

 hence the more dependent it becomes upon its neigh- 

 bours. 



Very early in the history of the development of an 

 animal, before any of the organs are formed, yet when 

 the number of cells is already large, these latter become 

 separated into layers. (Fig. 4.) 



FIG. 4. Transverse section to show the germ layers of a mole 

 embryo, X 265 diam. (Heape, Guam's Anatomy] ; ecto.^ 

 ectoderm ; ento.^ entoderm ; meso.^ mesoderm. 



The germ layers, as they are called, are composed 

 of cell-groups, which tend to show special characters, 

 according to the layer in which they occur. Ultimately 

 the layers give rise to three main subdivisions of the 

 body. From the ectoderm, or outer layer, are derived 

 the skin and the nervous system ; from the inner layer, 

 the entoderm, the alimentary tract with its appendages ; 

 and from the layer which lies between the two, the 

 mesoderm, are derived the muscles, the supporting 

 tissues, the reproductive system, and the circulatory 

 apparatus. 



It is for us to inquire how these portions grow, and 



