306 The Alligator and Its Allies 
the mesentery (ms), the section is quite deep dorso- 
ventrally. The continuation of the amnion (a) 
with the somatopleure is of course here evident. 
The most striking feature of the section is the 
marked projection of the Wolffian ridges, though 
no local enlargements of these vidges indicate the 
rudiments of the limbs. A large mass of Wolffian 
tubules (wt) is seen projecting into the upper part 
of the body cavity on each side; close to each of 
these masses is the posterior cardinal vein (pc), and 
between them is the large aorta (ao). The other 
structures are about as in the preceding section. 
Figure 16g represents a sagittal section of the 
anterior half of the body of an embryo of this or 
possibly a slightly younger stage of development. 
The three regions of the brain are clearly indi- 
cated, as well as the cavity of the spinal cord (sc). 
The roof of the hindbrain has been made too thick 
in the figure; it should be represented by a mere 
line. A little mesoblast is to be seen at places 
between the roof of the brain and the superficial 
ectoderm. A slight invagination of the epithelium 
(p), between the floor of the brain and the anterior 
end of the notochord, probably represents the 
beginning of the hypophysis. No indication of the 
paraphysis is yet to be seen. Extending from 
the region of the hypophysis to the posterior 
end of the section is the notochord (nt); it is much 
vacuolated and gradually increases in thickness 
toward the posterior, though its outline is quite 
