ORIGIN OF METAMERIC SEGMENTATION. 49 



from a phylogenetic standpoint that it is only the way in which 

 tlie animal develops, and that it is waste of time to attempt to 

 explain it. I cannot agree to accept such a view of any embry- 

 onic fact. If there is anything in the theory of evolution^ every 

 change in the embryo must have had a counterpart in the history 

 of the race, and it is our business as morphologists to find it out. 



I wish to point out that I am not discussing how the gas- 

 trula arose. I take as my basis the undoubted fact that gas- 

 trulse have existed, and I am trying to show that a two-layered, 

 gastrula-like animal was the ancestor of most living Metazoa. 



1 must, therefore, reject the view that the blastopore has no 

 ancestral meaning. 



What, then, is its ancestral meaning ? 



It seems to me that there is very strong evidence for the 

 view that it is homologous with the mouth of the Ccelenterata. 



In the first place the Coelenterate mouth either arises as a 

 result of invagination, the blastopore remaining as the mouth 

 (Cereanthus, Pelagia), or as the result of perforation. In the 

 Triploblastica similarly the blastopore either arises as a result 

 of invagination or as a perforation. The method of develop- 

 ment, therefore, coincides, and we thus have a strong reason 

 for regarding them as homologous. 



The second important point to be examined in determining 

 homologies is the relation to other important structures. The 

 relation of the Coelenterate mouth and the blastopore to the 

 alimentary canal and the nervous system can in most cases be 

 determined ; and in all cases in which it can be so determined, 

 it is the same. 



(1) The Coelenterate mouth and the blastopore resemble each 

 other in being the main communication by which the archen- 

 teric cavity or its rudiment communicates with the exterior. 



(2) They resemble each other in always being perforations 

 of the neural surface of the body. 



With regard to the first of these agreements nothing need 

 be said ; it is a fact of little importance, as there are many 

 other channels in the Ccelenterata through which the archen- 

 teron communicates with the outer world. The second agree- 



VOL. XXIV. NEW SEtt. D 



