298 The Alligator and Its Allies 
entoderm of even thickness. Dorsal to the foregut 
are three large blood-vessels, a median, and now 
single, dorsal aorta (ao), and a pair of cardinal veins 
(cv). The notochord (nt) is small and is flattened 
against the ventral side of the spinal cord (sc), 
which latter structure needs no special mention. 
The muscle plates (mp) are considerably elongated, 
so that they now extend ventrally to a point slightly 
below the upper angles of the body cavity. 
Figure 15e is through the middle region of the 
embryo, and, owing to the curvature of the body, 
is not an exact dorso-ventral section; this accounts, 
in part at least, for the unusual diameter in a 
dorso-ventral direction of the aorta (ao), which 
is very large in proportion to the other structures. 
The posterior cardinal vein is shown on the left, 
but not on the right. The relative sizes of the 
spinal cord (sc) and notochord (nt) are very 
different from what was seen in the preceding 
figure. In this section the spinal cord is consider- 
ably smaller than in the preceding, while the 
notochord is very much larger; in fact the noto- 
chord here seems abnormally large when compared 
to corresponding sections of other series. It 
is true, however, that while the spinal cord has 
been diminishing in diameter the notochord has 
been increasing. The spinal cord, notochord, 
and dorsal aorta are all so large that they are 
flattened against each other, the pushing in of the 
ventral side of the spinal cord being even more 
