THE DEVELOPMliNT OF THE JIOLE . 44(1 



neurenteric canal opens below to the cavity of the vesicle 

 (fig. 41). The arrangement of the layers at the front end of 

 the primitive streak may be shortly described, therefore, as 

 follows: — The epiblast and hypoblast meet and form the hind 

 •wall of the dorsal opening of the neurenteric canal; from the front 

 portion of this wall a tongue of mesoblast is projected forwards, 

 separated from the underlying hypoblast, but united with the 

 lateral mesoblast and with the epiblast in the middle line ; it 

 then joins the thickened axial hypoblast, and becomes freed 

 from the lateral mesoblast (fig. 39), while anteriorly to this 

 point the axial cells are continuous with both lateral hypoblast 

 and mesoblast. 



With regard to the structure of the remainder of the embryo, 

 the medullary groove is shallow and wide, and throughout its 

 length the axial hypoblast causes a swelling upwards of the 

 bottom of the groove. The lateral hypoblast and mesoblast 

 join the notochordal cells throughout the region where the 

 latter exist. 



The notochord is formed of cubical or columnar cells, and is 

 alternately in the form of an arch and a tube throughout the 

 whole length of the medullary groove (vide figs. 40 and 41). 

 Beyond the groove it becomes more flattened out (fig. 42), 

 and the arrangement there is similar to that described for 

 fig. 13. 



This tubular form of the notochord appears, to be very transi- 

 tory, as I have not met with it in any other embryo except in 

 that drawn in fig. 14, in which it is not either so definite or so 

 continuous. 



The sections through the area represented in fig. 14 show a 

 slightly different arrangement. Fig. 4.7 is taken through 

 nearly the same region as are the sections from which figs. 43 

 and 44 were drawn. The medullary groove is much the same 

 as is represented in fig. 43, but the notochord is less substan- 

 tial, and the single row of somewhat cubical cells of which it is 

 composed form an arch whose cavity opens into tliat of the 

 vesicle below. It will be obvious that the thickness of the 

 notochordal cells is much less than in fig. 41, and that the arch 



