314 THE EVOLUTION OF MAN. 



(protogaster, g) ; its simple opening, the primitive mouth 

 (jprotoatoma, o), is at once mouth and anus; and the two 

 cell-strata which compose its wall, are the two primary 

 germ-layers: the inner, the nutritive, or vegetative germ- 

 layer, is the intestinal layer (entoderma, i) ; and the outer, 

 covering layer, which, by means of its cilia, is also the agent 

 of motion, is the animal layer, or skin-layer (exoderma, e). 

 This highly important fact, that the gastrula appears as an 

 early larval condition in the individual development of the 

 most varied animals, and that this gastrula always exhibits 

 the same structure, and that the very differently developed 

 intestinal canals of the most varied animals, arises, onto- 

 genetically, from the same extremely simple gastrula- 

 intestine, this very important fact justifies, in accordance 

 with the fundamental law of Biogeny, two conclusions, 

 which involve important results, and of which one is general 

 and one special. The general conclusion is an inductive 

 one, and may be stated thus : The very variously formed 

 intestinal canal of all the different Intestinal Animals 

 has developed, phylogenetically, from one common and 

 extremely simple primitive intestine, from the intestinal 

 cavity of the Gastrsea, that primseval common parent-form 

 which is at the present reproduced, in accordance with the 

 fundamental law of Biogeny, in the gastrula. The second, 

 the special conclusion, which is connected with the former, 

 is deductive, and may be stated thus : The intestinal canal 

 in Man as a whole is homologous with the intestinal canal 

 in aU other animals ; it has the same original significance, 

 and has developed from the same rudimentary form.^se 



Before proceeding to trace the history of the develop- 

 ment of the human intestinal canal in detail, it will be 



