48 W. B. SCOTT AND HENRY F. OSBORN. 



they are still markedly continuous with the yolk hypoblast 

 cells lining the alimentary canal and their lower margin 

 arches upwards so as to form part of the lumen of the 

 canal. This bending around of the hypoblast, which in 

 Stage A was almost a straight line, into an arch of cells, 

 must be partly attributed to a mechanical cause, viz. the 

 rapid ingrowth of the mesoblast plates. Whatever the exact 

 cause of this change it is well to note that no vital altera- 

 tion has as yet taken place — the change is one merely of 

 position. Elsewhere the hypoblast shows no new features. 



Inasmuch as the interest in the hypoblast chiefly centres 

 around the development of the notochord we shall con- 

 sider the history of that organ by itself and complete the 

 hypoblast later. 



The Mesoblast. 



It is evident from transverse sections in the latter part of 

 Stage A (see PI. IV, fig. 4) that the lateral plates of meso- 

 blast have already attained a considerable thickness. At the 

 junction of the invagination with the yolk hypoblast they 

 are three or four cells deep, thinning out rapidly at the sides. 

 In the anterior sections they barely extend below the middle, 

 while behind they meet as a single layer of cells at the 

 bottom, thus encircling the hypoblast except in the axial 

 line above. 



The lateral downward growth of the mesoblast in Triton is 

 plainly not from the epiblast, for the epiblast has by this time 

 formed a distinctly bounded single layer. There remain two 

 modes in which it may in great part arise, {a) from the 

 hypoblast ; {b) independently of the hypoblast, from the yolk. 

 This is of course excluding the mesoblast in the region of the 

 alimentary canal which accompanies the process of invagina- 

 tion. If we consider, as we have reason to do from the analogy 

 of the Frog, that the cells bounding the yolk form the primi- 

 tive yolk hypoblast layer, we can only accept the former hypo- 

 thesis. In the anterior section of Stage A the cells bound- 

 ing the yolk below are as unquestionably hypoblastic as those 

 bounding it above and at the sides. In other words, the 

 hypoblast has formed as a distinct layer in contact with the 

 epiblast below, before the mesoblast has appeared below at 

 all. Moreover, at the sides, the down growth of the meso- 

 blast is preceded plainly by a splitting off of the outer portion 

 of the yolk hypoblast into large quadrate cells, and these in 

 turn are seen in the process of subdivision. Although this 

 growth seems to be at the expense of the hypoblast, it cannot 

 be considered to arise altogether independently of the down- 



