434 THE EVOLUTION OF MAN. 



passes through this gastrula-form, we are enabled to trace 

 its phylogenetic origin back to the Gastrrea. By tracing the 

 germ-histoiy of the two-layered germ still farther, we found 

 that, by fission, four secondary layers are produced from 

 the two original germ-layers. These have exactly the same 

 constitution and genetic significance in Man as in all other 

 Vertebrates. From the skin-sensory layer develops the 

 outer skin (epidermis) and the central nervous system, and, 

 probably, the kidney system. The skin-fibrous layer forms 

 the leather-skin (corium) and the organs of motion (the 

 skeleton and muscle systems). From the intestinal-fibrous 

 layer originates the vascular system and the fleshy wall of 

 the intestine. The intestinal-glandular layer, finally, forms 

 only the epithelium, or the inner cellular layer of the 

 intestinal-mucous membrane and of the intestinal glands. 



The manner in which these various organic systems 

 develop from the four secondary germ-layers, is, from the 

 very first, exactly the same in Man as in all other Verte- 

 brates. The germ-history of each separate organ afforded 

 proof that the human embryo takes exactly the same special 

 direction in its differentiation and formation, which, except 

 in Man, occurs only in the other Vertebrates. Within this 

 great animal tribe we then traced, step by step, and stage 

 after stage, the farther development which takes place in the 

 entire body as well as in all its several parts. This higher 

 development takes place in the human embryo in the form 

 peculiar to Mammals. Finally, we saw, that even within 

 this class the various stages of phylogenetic development, 

 which determine the natural classification of Mammals, 

 correspond throughout to the various stages of ontogenetic 

 formation through which the human embryo passes in the 



