42 Jenkinson, Germinal Layers of Vertebrates. 



The lower layer is an irregular sheet of scattered rounded 

 cells, not arranged at present in an epithelium, and is 

 constantly being reinforced by the addition of cells from 

 the nucleated yolk beneath (^Fig. 25, A). For the lower 

 layer I propose — in order to avoid any morphological 

 implications — to employ the term " paraderm," first 

 suggested by Kupffer. Between the paraderm and the 

 yolk is a shallow cavity, the subgerminal cavity. At one 

 point in the posterior margin of the embryonic shield the 

 upper layer and the paraderm are continuous ; this point 

 of fusion is termed by Will the primitive plate {^Fig. 

 25, A,//.)- 



The paraderm cells next arrange themselves in a flat 

 epithelium {^Fig. 25, B). At the same time a depression 

 makes its appearance in the primitive plate ; the anterior 

 margin of this depression is the dorsal lip of the blasto- 

 pore. 



Seen from the surface tlie dorsal lip presents the 

 appearance of a transverse groove at the hinder margin of 

 the embryonic shield. The groove rapidly becomes cres- 

 centic, the horns of the crescent grow back, meet, and fuse 

 behind the primitive plate, which now corresponds exactly 

 to the Amphibian or rather to the Gymnophionan yolk- 

 plug {^Fig. 24, A — C). A study of sections brings out the 

 similarity more clearly still. 



Beneath and in front of the dorsal lip there is produced 

 by invagination of the upper layer of cells a cavity which 

 rapidly increases in length until it reaches the anterior 

 end of the embryonic shield i^Fig. 25, C, D) ; the cavity is 

 proportionately broad {^Fig. 27, A). The roof consists of a 

 layer of columnar cells which at the dorsal lip turn over 

 in the ordinary way into the cells of the upper layer. The 

 floor is now, even in the region of the primitive plate, 

 perfectly distinct from the underlying paraderm. In 



