118 WALTEE HEAPB. 



separated from the lateral mesoblast, then reduced in thickness, 

 and finally converted (1) in the aaterior region into an arc 

 formed of a single row of columnar cells ; (2) towards the 

 central deepest portion of the medullary groove into a single 

 row of considerably flattened cells ; while (3) in the hinder 

 region it remains thickened and forms posteriorly the 

 anterior wall of the neurenteric canal, thus joining the 

 epiblast. 



During this stage (Stage d), the notochord is, throughout 

 its whole length, never actually isolated from the hypoblast, 

 but remains a portion of that layer, although an obviously 

 specialised portion ; it is in fact the remnant of the primitive 

 hypoblast (1. c). 



In this same paper (1. c. fig. 46) I described as a portion of 

 the notochord a short tube formed of columnar cells lying 

 below the medullary plate at the anterior end of the embryo. 

 I must here correct that error. This tube does not represent 

 the notochord solely, but constitutes the anterior end of the 

 alimentary tract (figs 11 and 13), and, as I shall show below, 

 the cells only of the dorsal portion of this tube give rise 

 eventually to the anterior end of the notochord. 



During Stages e and f the relations of the notochord 

 remain very much the same as they were during Stage d (figs. 

 14,15, and 17) ; it is noticeable, however, at the close of Stager, 

 that in the trunk of the embryo, where the medullary groove 

 is deep, the axial hypoblast has increased in thickness 

 (fig. 17). 



The deepening of the medullary groove towards the anterior 

 region which occurs during Stage a causes the notochord cells 

 situated there to be reduced in the same manner as they were 

 reduced in the central region during Stage d. Similarly the 

 axial hypoblast is . reduced in bulk in the posterior region of 

 the embryo, while in the central region, where the protovertebrse 

 are forming, there is a further increase in the size of the 

 notochord. 



At this stage of growth (Stage g) the notochord exhibits a 

 tendency to become separated from the hypoblast layer in the 



