The Development of the Alligator 281 
about forty sections, or one fifth of the length of the 
embryo. Their exact length is difficult to deter- 
mine because, while their anterior ends are blunt 
and sharply defined, they taper so gradually 
posteriorly that it is hard to tell just where they 
end. They apparently originate anteriorly and 
gradually extend toward the tail. In a slightly 
younger embryo the rudimentary Wolffian duct 
could be seen as a still smaller rod of cells extending 
posteriorly for a few sections, from the seventy- 
fifth section of a series of about two hundred. In 
the particular series under discussion the left 
rudimentary Wolffian duct was about one fifth 
longer than the right one. 
Figure 12f is just posterior to the head-fold of 
the amnion, passing, in fact, on the left side 
through the extreme edge of its lateral fold, which 
is shown as a upward bend in the ectoblast and 
somatopleure. 
The ectoblast (ec) shows the same remarkable 
thickening that was noted in the corresponding 
region of the preceding stage. The spinal cord 
(sc), notochord (né), aortz (ao), and entoderm (e7) 
need no special mention. The mesoderm seems to 
be separated by unusually wide spaces from both 
ectoderm and entoderm, and is made up of rather 
closely packed cells except around the aorte, 
where there seems scarcely enough tissue to hold 
these vessels in place. The body cavity (bc) is 
large, and a small myoccel (myc) is seen on the left. 
