60 WALTER HEAPE. 



14 (p. 59), excepting that the lateral cells below the epiblast 

 are not in such numbers as before, and the axial cells, instead 

 of being in the form of a tube, as in fig. 46, are only one row 

 deep, and are arranged as an arc, the bay of which opens 

 below (vide fig. 48). 



Behind the section (fig. 45) the groove also becomes shallower 

 and the notochordal cells thickei', terminating, as in former 

 specimens, in the front end of the primitive streak ; the latter 

 is, however, now cut off from the remainder of the streak, and 

 lies within the medullary groove, into which the dorsal pore of 

 the neurenteric canal now opens. 



The section of the groove in fig. 45 exhibits evidence in both 

 epiblast and hypoblast of an advanced growth on those drawn 

 in either figs. 43, 44, or 47, although the measurements of the 

 area are almost exactly similar to the one represented in fig. 14, 

 from which fig. 47 was taken. 



I do not propose to trace the development beyond this point, 

 but may briefly say, that after the stage just described the 

 medullary groove becomes much deeper, the epiblast of its wall 

 being thick, while the epiblast over the lateral portions of the 

 embryo is composed of only one layer of cubical cells. The 

 groove deepens first about the hinder portion of the anterior 

 third of the groove, and from there extends backwards and 

 forwards. 



The further development of the notochord takes place in the 

 same direction. The flattened notochordal cells seen in fig. 45 

 become slightly more rounded as the lateral hypoblast and 

 mesoblast sink to the level of the bottom of the groove, and 

 then the lateral hypoblast grows inwards, and a small bunch of 

 cells are isolated in the middle line and lie between it and the 

 now closed neural canal. 



The amnion is first formed, as I have stated, over the hind 

 end of the primitive streak, and from there grows forwards a 

 considerable distance before the head is covered by the ante- 

 rior fold. 



To recapitulate, we may conclude it is probable that 

 the region of the area in which the main portion of the 



