238 The Alligator and Its Allies 
dense mass of cells (ué), apparently the anterior 
end of the notochord in process of formation. 
Figure 32, still farther toward the blastopore, 
shows the medullary groove wider and shallower 
than in the more anterior sections. ‘The mesoderm 
(mes) is here a layer laterally distinct from the 
entoderm. In the middle line it is still continuous 
with the entoderm, and at this place it is the more 
dense mass of cells that may be recognized as 
the notochord (nt). It is evidently difficult to 
decide whether this group of cells (mt), which will 
later become a distinct body, the notochord, is 
derived directly from the entoderm or from the 
mesoderm, which is itself a derivative of the ento- 
derm. There is here absolutely no line of demar- 
cation between the cells of the notochord and those 
of the mesoderm and entoderm. 
In Figure 37 the ectoderm (ec) is nearly flat, 
scarcely a sign of the medullary groove appearing. 
The mesoderm (mes) is here a distinct layer, 
entirely separate from both notochord (ut) and 
entoderm (ez). The notochord is a clearly defined 
mass of cells, distinct, as has been said, from the 
mesoderm, but still closely united with the under- 
lying entoderm, which is much thinner than the 
ectoderm. This condition of the notochord, 
which is found throughout about one third of the 
length of the embryo, would give the impression 
that the notochord is of a distinctly entodermal 
origin. 
