64 THE EVOLUTION OF MAN. 



of the hollow formed the inner or vegetative layer, accom- 

 plishing the functions of nutrition ; the outer ceUs of the 

 covering formed the outer or animal layer, exercising the 

 functions of locomotion and covering the body. This 

 first and oldest process of differentiation is of such funda- 

 mental significance that it deserves the deepest thought. 

 When we consider that the body of the human being, 

 with all its diff'erent parts, and also the body of all other 

 higher animals, originates from these two simple primary 

 germ-layers, we cannot over-estimate the phylogenetic 

 significance of the gastrula. For in the quite simple primi- 

 tive intestine, or the primitive intestinal cavity of the 

 gastrula and its simple mouth-opening, the first real organ 

 of the animal body, in a morphological sense, is gained; 

 the earliest genuine organ, from which all the other organs 

 have differentiated at a later period. The whole body of 

 the gastrula is really only a " primitive intestine." 



We have already pointed out the remarkable agreement 

 between the palingenetic gastrula-forms of animals of the 

 most diverse classes ; of Sponges (Fig. 174, A), Polyps, 

 Corals (Fig. 171, /), Medusa, Worms (Fig. 175, B) Star- 

 animals (Echinoderma, G), Articulated Animals (Arihro- 

 poda, D), Soft-bodied Animals (MoUusca, E), and Verte- 

 brates (F). All these various forms of the palingenetic 

 gastrula are much alike, and are only distinguished by such 

 unessential and subordinate peculiarities, that the systematic 

 zoologist, in his " natural system," could only represent them 

 as different species of a single genus. The various kenoge- 

 netic gastrula-forms which have been described were also 

 referable to that original jjalingenetic form (vol. i. p. 231). The 

 gastrula proved to be a germ-form common to all classes of 



