Manchester Mei?ioirs, Vol. I. (1906), No. 3. 65 



digestive tract of these. Two distinct processes, according 

 to this author, have hitherto been confused under the 

 head of gastrulation. One — which is alone represented 

 in Invertebrate forms — is an invagination of the primary 

 endoderm into the segmentation cavity ; the other — 

 pecuHar to the Vertebrates — is the overgrowth and in- 

 growth of a ' dorsal plate ' of cells (the roof of the 

 archenteron) produced entirely from the primary ecto- 

 derm and taking place most actively at the dorsal lip of 

 the blastopore. The ectodermal dorsal plate gives rise 

 to notochord and mesoderm, while the alimentary tract is 

 derived entirely from the invaginated endoderm cells. It 

 must be pointed out that, quite apart from the difficulties 

 of the Gastrula theory, of which it is of course a modifica- 

 tion, Lwoffs hypothesis is very seriously contradicted 

 by the fact that the roof of the archenteron — his ' dorsal 

 plate' of 'ectoderm' — in many cases either contributes to 

 or forms the whole of the digestive canal. To argue, as 

 Lwoff actually does, that in these cases primary endoderm 

 ceils have found their way into the roof of the archen- 

 teron is, it is superfluous to point out, destructive of the 

 universality of the ' ectodermal ' origin of the latter, for 

 which, as a matter of fact, a good deal might be said. 



Hubrecht's theory resembles that of Lwoff in attribut- 

 ing to the ' dorsal ' or ' notochordal ' plate the sole function 

 of notochord and mesoderm formation ; but differs from it 

 in seeing in the segmentation cavity, with which, as this 

 embryologist urges, the archenteron in all cases becomes 

 confluent, the primitive Coelenterate gut, the archenteron 

 itself being then regarded simply as an ectodermal stomo- 

 daeum. 



Now underlying the Gastrula theory and its modifica- 

 tions there are two fundamental assumptions, assumptions 

 which must be vindicated if the theory is to be maintained. 



