410 WALTER IIl'L\PE. 



live streak and the structure of the neurenteric canal at this 

 stage of growth will readily be understood by a glance at the 

 drawing of the surface view of an embryo (fig. 15), and com- 

 paring it with the diagrammatic longitudinal section in fig. 50 

 and the transverse sections in figs. 37 to 41. The latter arc 

 taken from an embryo of an ago between that of fig. 13 and 

 fig. 14, and the walls of the medulUiry canal do not yet enclose 

 tlic front end of the primitive streak, althougli tlie latter is 

 already placed between them. 



The longitudinal section is taken from a younger embryo. 



The dorsal hinder opening of the neurenteric canal (figs. 15 

 and 37) is formed by an involution of the epiblastin the middle 

 line at the head end of the ])rimitivc streak, and the separation 

 of the lips of the involuted layer. 



The passage so formed enters the mass of mesoblast cells, 

 budded off from epiblast and liypoblast in this region, as it 

 does in the embryo of which fig. 13 is a drawing; but it does 

 not end there, it travels forwards almost parallel to the plane 

 of the layers, and is seen eight sections further forward (fig. 38) 

 as a canal within the axial mass of mesoblast, which we have 

 invariably seen to be projected anteriorly from the front end of 

 the primitive streak. There is no doubt the cells surrounding 

 the canal at tliis point are mesoblast cells; they are continuous 

 with the epiblast in the middle line and with the lateral 

 mesoblast, and there is a distinct layer of hypoblast below them 

 (compare fig. 38, and fig. 50 immediately in front of ^;. sk'); 

 gradually, however, the canal dips downwards, and as this 

 prolonged cord of mesoblast joins anteriorly the axial hypo- 

 blast, the walls of the canal also there, some sixteen sections in 

 front of fig. 37, become hypoblastic (vide fig. 39). Here the 

 lateral mesoblast does not join the thickened axial hypoblast, 

 which is continuous with the lateral hypobhist only. 



Three sections further on the axial cells become continuous 

 with both lateral mesoblast and hypoblast. The lower wall of 

 the canal now shows signs of becoming thinner (fig. 40), and 

 five sections beyond this, that is, twenty-four sections from the 

 liind opening, it becomes divided in the median line, and the 



