20 AETHUR DENDY. 



ever, that part of the niesoblast in this region is derived from 

 the forward growth of the primitive streak described in the 

 preceding stage. 



Passing backwards from the last section the medullary 

 groove is seen to disappear, and give place to a continuous 

 flat medullary plate, composed of radially elongated, columnar 

 epiblast cells, and extending for some distance. As it ap- 

 proaches the blastopore the epiblast thins out in the middle 

 (fig. 11), and a groove (P. G.) appears on its upper surface, 

 which can be traced back almost to the blastopore. This 

 groove has prominent lips in the hinder part of its course 

 (fig. 12, P. G.). It probably corresponds to the primitive 

 groove of the chick. 



For some distance in front of the blastopore the mesoblast 

 forms a very dense mass of cells, which appear to be rapidly 

 proliferating, and this is continued as a thick layer right 

 across the middle line, and also outwards, on either side, as 

 two thin lateral sheets (figs. 11, 12). This mass of cells is 

 clearly derived from the forward prolongation of the primitive 

 streak described at Stage C (compare fig. 4). It is perfectly clear 

 that the embryonic mesoblast originates from two sources, viz. 

 (1) from the original lower-layer cells of the area pellucida after 

 separation of the definite hypoblast (compare fig. 8), and (2) from 

 the cells of the primitive streak. The limits between the two 

 different kinds of mesoblast I have not been able to determine. 



Fig. 12 represents a section just in front of the blastopore, 

 through the primitive streak. It will be noticed that the 

 columnar or prismatic epiblast of the embryo passes into the 

 undifferentiated mass of the primitive streak a little way in 

 front of the blastopore, so that the section under consideration 

 shows no differentiated epiblast on the dorsal surface at all. 

 The mid-dorsal line is occupied by the primitive groove (P. G.) 

 with its prominent lips. Beneath this, deeply embedded in the 

 dense mass of the primitive streak, lies a small cavity {N. JE?i.). 

 This is the cross-section of the anterior ventral end of the 

 neurenteric canal, which passes downwards and forwards from 

 tlie blastopore, but does not yet open below. 



