ORIGIN OF THE ARCHENTERON. 2:^7 



of the primitive band it gradually becomes shortened, and 

 by the time this is complete the dorsal part of its epiblast 

 is drawn back on the dorsal surface of the yelk and termi- 

 nates at its posterior extremity, whilst the preoral lobes 

 increase until they cover the anterior third of the yelk on its 

 dorsal as well as its ventral aspect. 



3. ORIGIN OF THE ARCHENTERON, SOMATOPLEURE AND 

 CCELOMIC SACS. 



Origin of the Hypoblast. — According to most embryologists 

 the archenteron in all the Metazoa originates by gastrula- 

 tion, yet if the accepted origin of the hypoblast in Insects 

 be correct, it appears to me it must be admitted that they 

 conform in no way with the rest of the animal kingdom, but 

 possess an alimentary canal developed in a manner which 

 cannot be regarded as gastrulation. 



I have already referred to two so-called gastrula stages (p. 240), 

 but neither one nor the other can be said to be a modification 

 of a typical gastrula. As the gastrula of Amphioxus or of an 

 Echinoderm is regarded as typical, it is apparent that a true 

 hypoblast must originate from the external covering of the 

 blastocele, and may be said to be a mere invagination of the 

 epiblast, or rather of the undifferentiated blastoderm, which 

 becomes both epi- and hypoblast. 



It is admitted that the process of gastrulation is much modi- 

 fied by the presence of a large food-yelk. In those Vertebrates 

 in which such a yelk is present the epiblast is supposed to over- 

 grow the hypoblast ; but the true hypoblast is still a portion of 

 the discoid blastoderm. According to Selenka, the primitive 

 rudiment of the vertebrate archenteron is always an invagina- 

 tion of the primitive blastoderm, although, except in some fishes 

 (Teleostei), it is no longer hollow beyond the anterior ex- 

 tremity of the primitive streak, which Selenka regards as the 

 homologue of the posterior part of the alimentary canal. 



The accepted view that the mesenteron in Insects is developed 

 from cells which are formed in the yelk beneath the blastoderm, 



17 



