442 WALTER IIEAPE. 



is not so completely tubular. The lateral mcsoblast is not 

 continuous with the axial hypoblast (notochordal) cells in the 

 middle and deeper portion of the groove, but such is the case 

 both further forwards and backwards. 



Immediately beyond the anterior end of the groove the flat- 

 tened epiblast is thickened to form the medullary plate, and 

 below it the arched notochordal cells form a complete tube, 

 the structure of which is similar to that already described 

 excepting that the lower wall of the tube is thicker. 



Fig. 46 is a section through this region of fig. 14, the dark 

 streak seen in surface view being accounted for by this thick- 

 ened mass of notochordal cells ; it is the only portion of the 

 notochord in the anterior region which remains thickened at 

 this stage of growth. The relations of the layers at the hind 

 end of the area are the same as are described on p. 



At a stage slightly older than that represented in fig. 14 

 the medullary groove is still deeper. A section (fig. 45) taken 

 through about the same region as are those drawn in figs. 43 

 and 44 demonstrates this. The epiblast now exhibits a further 

 change, that portion of it forming the walls of the groove are for 

 the most part but slightly thicker than heretofore, but imme- 

 diately on each side it becomes suddenly considerably thicker, 

 and then gradually becomes thinner again towards the boundary 

 of the area, where it is turned up to form the commencing 

 amnion. 



The mesoblast is here completely separated from the axial 

 cells, being rounded oflf at the sides bordering the medullary 

 groove, and at the edges it is split to form splanchnic and 

 somatic layers. The hypoblast is continuous across the area, 

 the axial portion exhibiting no increase in thickness to that 

 situated laterally ; the former being forced by the deep medul- 

 lary groove into a bow projecting into the vesicle below. 



In front of the point from which the section is taken, the 

 groove first becomes narrower, and then more shallow and 

 wider, the notochordal cells becoming at the same time thicker 

 and continuous with the lateral mesoblast. In front of the 

 groove the relations are similar to Avhat were described for fig. 



