THE DEVELOPMENT OF THE MOLE. 185 



median invagination (figs. 44, 45). These grooves are formed 

 along the anterior border of the first visceral arch. The epi- 

 blast and hypoblast are in close contact along the whole of the 

 V-shaped groove, but become actually fused posteriorly at the 

 apex, where the mouth will eventually be formed (compare 

 figs. 44, 45, and 49). 



It will be seen by the foregoing description that the mouth 

 is formed somewhat behind the anterior end of the fore-gut at 

 the apex of a V-shaped groove on the ventral surface of the head, 

 the diverging limbs of which groove are directed forwards. 

 The section of the gut which is placed anteriorly to the 

 mouth is identical with the blind tube first formed by the 

 folding-off of the embryo from the yolk-sac, and this anterior 

 diverticulum exists for some time after the ventral enlargement 

 of the gut towards the external groove. 



These facts appear to indicate that a more primitive mouth, 

 the terminal position of which is indicated by the primary 

 anterior diverticulum of the fore-gut, has been replaced by a 

 secondary formation, the paired origin of which is rendered 

 possible by the two converging grooves in the epiblast of the 

 ventral surface. 



If these observations are correct, they must be considered to 

 some extent confirmatory of Dr. Dohrn^s theory of the paired 

 origin of the existing mouth of the Vertebrata, but I would 

 suggest that such evidence cannot be used as argument for the 

 paired formation of the primitive Vertebrata mouth, the 

 terminal position of such being exceedingly probable. 



As in the earlier stage, the cells forming the dorsal wall of 

 the fore-gut are throughout thinner than those lining the 

 remainder of the cavity, and in the posterior section of its 

 length are much flattened ; on the other hand the cells of the 

 ventral wall, the lateral horns, and the outgrowths to form the 

 visceral clefts, are cubical or even columnar in form. 



The Notochord. — The notochord, as I have before described 

 (No. 8, figs. 37 — 48), is a hypoblastic structure and is primi- 

 tively in connection with the hypoblast and the lateral plates 

 of meso blast of the embryo. During Stage d it becomes first 



