130 



EMBRYOLOGY. 



one, the outer germ -layer being followed by the middle layer, and this 

 by the inner germ-layer. 



ak 



mk 

 ik 



l~ ak 



Fig. 92 A and B. Cross sections through the germ-disc of a Selachian. Copy after BALFOUR'S 



Monograph, PI. IV., Fig. 8a, and PI. IX., Fig. la. 

 Only the left half of section A is represented. 

 ak, Outer, ik, inner, mk, middle germ -layer ; ch, chorda ; ;np, medullary plate ; <l , yolk. 



The conditions in detail assume in Selachians, Birds, and Mammals 

 the forms indicated by the accompanying figures (92-95). 



In the Selachians the medullary fold is well marked in cross 

 sections (fig. 92 A mp). Beneath it there lies, as in Amphioxus and 

 Triton, only a single layer of tall cylindrical cells (ch), the funda- 

 ment of the chorda ; laterally this merges into a many-layered mass 

 of small cells, which is soon divided by means of a fissure into two 

 distinctly separated lamellae into the middle layer (mk), composed 

 of small polygonal cells, and into the inner layer (ik"), which here 

 consists of a single layer of tall columnar cells. At the point in- 

 dicated by a star, the fundament of the chorda and the middle 



and inner germ-layers 

 are continuous with one 

 another. At a later 

 stage (fig. 92 B) a se- 

 paration of the three 

 fundaments takes place, 

 as in Triton, and we 

 then have (1) a round 

 chordal rod (ch), which 

 has been formed by in- 

 folding in the manner 

 previously described ; (2) 

 at either side of it the small-celled mass of the middle germ-layer 

 (mk), divided into halves by the chorda ; (3) the inner germ-layer 

 (ik), the halves of which, separated in the previous stage, are 

 now growing under the chorda, and are about to fuse into a single 

 layer. 



ak 



ch 



Fig. 93. Cross section through the blastoderm of a Chick 

 in which the first traces of the chorda and the medullary 

 furrow are to be seen, after BALFOUR AND DEIGHTON. 



The section passes through the fundament of the chorda 

 in front of the primitive streak. The part of the 

 section at the right of the fundament of the chorda is 

 not figured. 



ak, Outer, ink, middle, ik, inner germ-layer ; ck, fundament 

 of the chorda. 



