The Development of the Alligator 245 
secondary folds of the anterior end of the embryo 
and the just forming medullary folds of the pos- 
terior end, so that it is impossible to say whether 
the thickenir~ of ectoblast in this figure should be 
called secondary folds or medullary folds. As a 
matter of fact, the secondary folds become, of 
course, the anterior ends of the medullary folds. 
The mesoblast (mes) is here of considerable extent, 
and its entodermal origin is beyond doubt, though 
not well shown in the figure. 
Figure 6f is about one sixth of the length of the 
embryo posterior to the preceding. The medul- 
lary thickening of the ectoderm (ec)is still marked 
and the shallow medullary groove (mg) is fairly dis- 
tinct. The entoderm (ez) is medially continuous 
with both mesoderm (mes) and notochord (nt), 
though these two tissues are otherwise distinct 
from each other. 
Figure 6g is nearly one third the length of the 
embryo posterior to the preceding and passes 
through the posterior third of the embryo. The 
medullary thickening of the ectoderm (ec) is 
marked, but shows no sign of a medullary groove; 
in fact, the median line is the most elevated region 
of the ectoderm. The notochord (n#) is larger 
in cross-section than in the more anterior regions. 
It is still continuous with the entoderm (ez) and 
is fairly closely attached to, though apparently 
not continuous with, the mesoderm (mes) on each 
side. 
