280 The Alligator and Its Allies 
mesoblastic part of the amnion (a). The amnion 
may be traced through about 130 of the 200 sec- 
tions into which this embryo was cut. 
Figure 12e is nearly one fourth the length of the 
embryo posterior to Figure 12d; it is approximately 
in the middle region. The diameter of the embryo 
has been gradually decreasing until now it is very 
much less than in the head region. The section 
being behind the head-fold the entoderm (em) is 
nearly flat and the enteron is quite unenclosed. 
The canal of the spinal cord (sc) is smaller in 
proportion to the thickness of its walls, and the 
notochord (nt) is somewhat larger than in the 
preceding sections. In proportion to its extent, 
the ectoderm is very thick. Under the notochord 
the dorsal aorte (ao) are seen as two large, round 
openings in the mesoblast. On the left side the 
section passes through the center of a somite and 
shows a small, round myoccel (myc). The meso- 
blastic layer of the amnion (so) is distinct through- 
out from the ectoblastic layer (a). 
The most important structures to be here noted 
are the first rudiments of the Wolffian ducts (wd). 
They are seen in the present section as lateral ridges 
of mesoblast projecting outward and upward 
toward the ectoblast, which suddenly becomes 
thin as it passes over them. These ridges or cords 
of mesoblast are as yet quite solid. They arise 
suddenly at about the eightieth section of the 
series of two hundred and may be traced through 
