242 The Alligator and Its Allies 
for the anterior part of the medullary folds that 
I have made use of both Figures 4 and 5. They 
were made from very perfect specimens, and the 
sections of both of them, and of the specimen 
from which Figure 6 was drawn, proves that the 
structure is what it is indicated to be in surface 
appearance. That is, the transverse sections pos- 
terior to the V, in the embryos shown in Figures 4 
and 5, show the medullary groove and the medul- 
lary folds; the several sections passing through the 
apex of the V show neither groove nor folds, but 
only a median thickening; and in front of the point 
or apex of the V the successive sections discover 
a gradually widening groove between the arms, 
which is also much deeper than the shallow groove 
found posterior to the V. While I have not seen, 
and from the nature of the conditions one cannot 
see, the change actually proceeding from the form 
of Figure 5 to that of Figure 6, still the explanation 
given appears to be the only one possible”’ (17). 
A somewhat extended series of transverse sec- 
tions of an embryo of about this age is represented 
in figures 6a—-1. 
Figure 6a is a section through the head-fold; it 
passes through the extreme anterior end of the 
secondary folds (sf) that were described, in surface 
view, above (Figs. 5 and 6). The section was not 
quite at right angles to the long axis of the embryo, 
so that the fold on the right was cut farther toward 
its anterior end than was the fold on the left. 
