. Manchester Memoirs, Vol. I. (1906), No. II. 61 



Amniota of that portion of the edge of the y\namnian 

 blastoderm, which in the Gymnophiona is excluded from 

 participation in the lip of the blastopore, is to be sought 

 not in the edge of the blastoderm but in the margin of 

 the embryonic shield. 



It is interesting to notice that in various Anamnia — 

 Petromyson and Rana — the same union of the archenteric 

 and segmentation cavities which is observed in Gymno- 

 phiona and Amniota also occurs. 



Before concluding this discussion I ought to glance 

 at the view which Oscar Hertwig has recently published. 

 Without attempting any more serious solution than is 

 contained in the hint that the clue to the problem must 

 be sought in the Gymnophiona, Hertwig expresses it as 

 his opinion that the separation of the Amniote blastoderm 

 into upper and lower layers represents in a measure a 

 process of gastrulation though the cavity of invagination 

 is entirely absent. To the archenteron of the Amniota, 

 on the other hand, he refuses a complete homology with the 

 similarly named cavity in the lower forms ; it is, according 

 to him, merely a ' Mesodermsackchen ' involved solely 

 in the production of the middle layer and the notochord. 

 Although it is true that the homology in question is — if 

 destiny as well as origin be taken into consideration — 

 usually, though not always, an incomplete one, still for 

 the purposes of comparison of one and the same stage in 

 development throughout the series it may be regarded as 

 complete ; for the upper layer contains in the floor of the 

 invagination cavity and the primitive plate cells which 

 can and must be compared with the cells which line the 

 floor of the archenteron and form the yolk-plug in the 

 Anamnia. 



