438 WALTEK HE APE. 



There was no indication, as far as I could see from the surface, 

 of a dorsal opening to a neurenteric canal in this embryo. The 

 growth from the front end of the primitive streak is similar to 

 what has already been noticed in younger embryos (fig. 31 and 

 p. 431). 



Fig. 15 is a drawing of the hind end of the medullary groove of 

 a still older embryo. Here the walls of the iiind end of the groove, 

 hitherto widely separate, have joined each other, and have en- 

 closed within the groove the front end of the primitive streak, 

 and with it the hinder dorsal opening of the neurenteric passage. 

 From the front end of the primitive streak a prolongation is 

 sent forward similar to that seen in fig. 14. 



The walls of the groove are now distinct. 



The structure of the groove of Buch an embryo as that drawn 

 in fig. 13 is represented in transverse section in fig. 43. The 

 plate of epiblast is thin where it is grooved, on each side, 

 however, becoming about double the thickness, and then 

 gradually thinning off until it is only a single layer deep 

 at the edge of the area ; here it is curved upwards, and thus 

 indicates the commencement of the amnion. 



The cells underlying the epiblast in this region are divided 

 in the same manner as we have seen are those of a much 

 younger embryo (stage c, p. 434), into (1) lateral plates of 

 mesoblast and hypoblast, and (2) an axial mass of cells, the com- 

 mencing notochord showing no differentiation into those 

 layers. 



It will be noticed, however, that the axial cells are consider- 

 ably more isolated from the lateral masses than heretofore, 

 although still continuous with the latter. The lateral meso- 

 blast is thick, and at the edge of the area becomes divided into 

 two layers, which are the future somatic and splanchnic 

 mesoblasts. 



These relations remain the same throughout the medullary 

 groove, excepting that at the posterior end the axial noto- 

 chordal cells become thicker and join a forward growth from 

 the front end of the primitive streak, while at the anterior end 

 the groove widens, and all the cells underlying the epiblast 



