248 The Alligator and Its Allies 
determined because of a break in the series at this 
point. The section passes through the posterior 
limit of the head-fold. The medullary groove 
(mg) is very deep and comparatively wide; around 
its sides the germ layers are so closely associated 
that they may scarcely be distinguished. Ventral 
to the medullary groove the foregut (fg) is seen 
as a crescentic slit. 
Figure 7d is about a dozen sections posterior to 
the one just described and is about one seventh 
the length of the embryo from the anterior end. 
The embryo is much more compressed, in a dorso- 
ventral direction, and the medullary groove (mg) 
is correspondingly more shallow. Where the ecto- 
derm (ec) curves over to form the medullary folds 
it becomes much more compact and somewhat 
thicker. The notochord (n/) is large and distinct, 
but is still fused with the entoderm (en). The 
mesoderm (mes) forms a well-defined layer, en- 
tirely distinct from both the notochord and the 
entoderm. From this region, as we pass caudad, 
the size of the embryo in cross-section gradually 
decreases and the medullary groove becomes more 
shallow. 
Figure 7e is about one third of the length of the 
embryo from the posterior end, and is only a few 
sections from the caudal end of the medullary 
groove. The ectoderm (ec) is much thinner than 
in the preceding figure and the medullary groove 
(mg) is much more shallow. The notochord (é) 
