Manchester Memoirs, Vol. I. (1906), No. 3. 41 



AMNIOTA. 



Whereas in the Anamnia the blastoporic lip is formed 

 at the edge of the blastoderm in the Amniota the blasto- 

 pf)re lies wholly within the latter. The Amniote blasto- 

 derm^ consists of two layers an upper and a lower 

 (fiequently termed epiblast and hypoblast and identified 

 with the definitive ectoderm and endoderm). Well within 

 the margin of this blastoderm a blastopore (primitive 

 groove) is formed leading into an archenteron ; concomi- 

 tantly the material for the germ-layers is laid down. All 

 these structures are in many cases derived in the first 

 instance solely from the upper layer, though eventually a 

 connection with the lower layer is set up. The edge of 

 tlie blastoderm, which is entirely independent of the 

 blastopore, grows steadily over the surface of the yolk 

 finally enclosing it at the vegetative pole. 



The whole process is far clearer in the Reptiles than 

 in either of the other two groups. They will accordingly 

 be considered first. 



REPTILIA. 



The principal authorities for the early development of 

 the Reptilia are Will, Wenckebach, Mehnert, Mitsukuri, 

 and Ballowitz. The best account is perhaps Will's 

 description of the formation of the layers in Platydactylus, 

 a Gecko. 



There is distinguishable in the blastoderm at the close 

 of segmentation a circular or oval area placed excentri- 

 cally towards the posterior end ; this area is the embryonic 

 shield i^Fig. 24). The blastoderm consists of two layers, 

 an upper and a lower ; the upper layer consists of 

 cylindrical cells in the embryonic shield, of flat cells in the 

 surrounding region ; below it is the segmentation cavity. 



'•*See Note 7, page 18. 



