THE DEVELOPMENT OP THE MOLE. 61 



embryo is formed is derived by a forward growth. The hind 

 knob of the primitive streak is lost, and the pyriform hind end 

 of the area becomes shortened and widened. The medullary 

 groove appears first as a wide shallow groove in the region 

 adjoining the head of the primitive streak, from which point it 

 extends forwards. 



The changes undergone by the axial hypoblast are some- 

 what complicated. At first the cells of which it is composed are 

 numerous, they then become fewer, and are arranged first as a 

 flattened then as an arched plate, which may or may not be 

 completely closed in to form a tube. Later on the arched 

 plate becomes flattened out again by the deepening of the 

 groove, and the notochord is represented by a thin layer of 

 flattened cells which, as the lateral mesoblast and hypoblast 

 sink down to a level with the bottom of the groove, become 

 again more cubical in form. Eventually the lateral hypoblast 

 grows in from the sides and the axial cells are separated off as 

 a notochord. 



The isolation of the axial cells from the lateral mesoblast 

 takes place, as does the separation of the notochord from the 

 hypoblast, from about the middle of the embryo backwards 

 and forwards. 



The same may be said for the medullary groove, which is 

 first formed from behind forwards ; its conversion into a canal 

 takes place from about the middle towards the hind and front 

 ends. 



The neurenteric canal is complete, opening at first dorsally at 

 the head end of the primitive streak, and between the latter 

 and the medullary groove, but eventually becoming enclosed 

 within the groove and opening at the bottom of its hinder end. 



The canal travels forwards in an anterior growth of meso- 

 blast from the head of the primitive streak, and enters a thick- 

 ened axial mass of hypoblast, from which it opens downwards 

 to the cavity of the vesicle. 



The amnion is first formed over the hind end of the embryo, 

 and only at a considerably later period envelops the front end 

 by a separate formation. 



5 



