3 8 GENESIS OF MAN. 



is the difficulty both in finding the true homologue in man and 

 the higher vertebrates generally, of the Gastrula of the Ascidian 

 and Amphioxus, and also in finding any good systematic represen- 

 tative of the ancestral Gastraea. 



The rest of the history of the ontogenetic development of man 

 is the history of the differentiation of the two primary germina- 

 tive layers. The Gastrula stage has furnished, in these two layers, 

 the raw material for the entire future structure. By watching the 

 progress of growth in exoderm and entoderm, the successive tis- 

 sues of every part of the body may be traced to the highest degrees 

 of specialization. From this point of view that stage possesses an 

 interest far exceeding that of all those that have preceded it : for in 

 it is found the first truly specialized organ. That organ is the sto- 

 mach. The two essential functions of life are nutrition and repro- 

 duction. The one is the promoter of ontogenetic, the other of 

 phylogenetic development. But, as we saw in the cell, these two 

 functions are originally one, and that one is nutrition. Repro- 

 duction appears here as a mere continuation of nutrition. Nutrition 

 goes on to the limit of growth, when division takes place. Nutri- 

 tion is commuted into reproduction. Generation is phylogenetic 

 nutrition ; a truth which we should never have reached except by 

 the study of the lowest organisms. Nutrition, therefore is the one 

 essential function of life. The organ of nutrition is the stomach. 

 How significant, and yet how reasonable, that in the course of 

 development the first specialized organ should be the stomach, and 

 that the first creature possessing a specialized organ should consist 

 wholly of a stomach ! Such a form is the Gastrula ; such a crea- 

 ture was the Protascus ; and such is the hypothetical Gastraea of 

 Haeckel. 



The sixth stage of the ontogenetic development gives the human 

 embryo the form and organization of a worm. Our moral and 

 religious teachers have from time immemorial delighted in remind- 

 ing us that we were but "worms of the dust." They should thank 

 science for demonstrating that they were right. We might almost 

 give them credit for an inspirational insight, did they not render 

 their sincerity questionable by the indignation they evince when 

 told that in the same sense that we are worms, we are also apes. 



The first important step in the progress of embryonic develop- 

 ment, after leaving the Gastrula-stage, is the formation of two 



