214 RICHARD ASSHETON. 



neural tube (NP.). So also, but less distinctly, in the earlier 

 stage (fig. 8). 



In the stages earlier than this it is not perceptible in surface 

 views, but may be easily recognised at its first appearance in 

 stages like figs, 4 and 4 a. 



At this time — its first appearance — it shows itself as a slight 

 tendency to a more rapid growth of the hypoblast immediately 

 underlying the anterior and lateral edges of the embryonal 

 area. 



Figs. 17 and 18 show the anterior edge cut at PC. 



Sections 13 and 14, if continued to the edges, would show the 

 same slight thickening of the hypoblast. 



Figs. 35 — 38, PI. 21, show the subsequent history of this 

 thickening. 



Fig. 35 is a transverse section through the lateral edge of the 

 anterior part of the embryonal area of a stage between those 

 represented in figs. 4 and 5. It is in front of the primitive 

 streak area. There is as yet in this region no mesoblast. The 

 hypoblast {HY.) shows, however, signs of increased activity, and 

 is thicker than before. This is specially the case at the 

 edge of the thickened epiblast. 



Fig. 36 is a slightly older specimen, still rather younger 

 than fig. 7. Here the hypoblast {HY.) is thickened over the 

 whole area, extending under the thickened epiblast, but more 

 especially so at the edge of the epiblast. 



A certain number of cells are to be seen lying between the 

 epiblast and hypoblast, marked PC. in my drawing. These 

 I believe to have been budded off in situ from the hypoblast 

 (HY.). Cells are budded off from the hypoblast all over this 

 area of the anterior part of the embryonal disc, but more 

 thickly at this region round the edge of the thickened epiblast 

 than elsewhere. Fig. 37 is a section from the same region of a 

 later stage, a stage with two mesoblastic somites (vide fig. 9). 



There seems to be here still a slight proliferation of cells 

 from the hypoblast of tliis region, but not so great as before. 

 The cells formerly budded off, which we can call mesoblast, 

 have become arranged so as to leave a slight cavity between 



