48 Jenkinson, Germinal Layers of Vertebrates. 



a median notochord and lateral mesoderm, and a lower 

 which becomes the epithelium of the gut. Such dis- 

 crepancies recall the similar diversities amongst the 

 Anamnia. 



AVES. 



The conditions observable in the Birds are very 

 readily derived from and very easily understood in the 

 light of those which obtain in the Reptiles. 



The primitive groove is simply a laterally compressed 

 blastopore. In front of the anterior end — the dorsal lip — 

 the notochord is produced i^Fig. 30, A) ; to right and left 

 of the notochord are the sheets of mesoderm which, 

 springing from the sides — the lateral lips — of the primitive 

 groove i^Fig. 29, B), are continued into one another at its 

 posterior end, where there may be an actual ventral lip 

 {^Fig. 30, B). The archenteric cavity has, however, in most 

 cases disappeared ; between the sides of the primitive 

 groove, which, even in those cases in which it is most 

 reduced, exhibit the characteristic structure of blastoporic 

 lips {^Fig. 29, B), is merely a mass of cells — the representa- 

 tive of the yolk-plug — fused with the paraderm beneath ; 

 and the belated 'neurenteric canal' is the sole vestige of the 

 archenteron and the communication which we have seen 

 to be effected between it and the sub-germinal cavity in 

 the Reptiles. 



In some cases, however, described by Schauinsland^ 

 the archenteric cavity is better developed, and the blasto- 

 pore quite similar to that of the ReptiIia(/^^>. 30). The primi- 

 tive streak and groove invariably originate in the upper 

 layer (^Fig. 29, A), fusion with the paraderm being merely^ 

 secondary ; only after the germ-layers have been formed, 

 can the upper layer be described as ectoderm. The 

 paraderm always gives rise to the alimentary tract ; 

 whether it also provides cells for the anterior extension of 



