316 The Alligator and Its Allies 
surface view of the embryo. ‘The digestive tract 
is cut through its extreme posterior end, in the 
region that may be termed the cloaca (cl), for into 
it at this point the Wolffian ducts open (wdo). 
As the narrow cloacal chamber is followed toward 
the tail, it becomes still smaller in diameter, and 
the ventral depression or cleft seen in this figure 
gradually becomes deeper until its walls are contin- 
uous with the ectoderm that covers the ventral pro- 
jection of mesoderm between the hind legs; no 
actual opening to the exterior is present, however. 
There is a space of about twenty-five or thirty 
sections (in a series of eight hundred) between the 
posterior ends of the Wolffian bodies and the 
cloacal openings of the Wolffian ducts. The 
body cavity (bc) and the posterior cardinal veins 
(pc) are very small in this region, as might be 
expected. 
STAGE Xv 
Ficure 18 (PLATE XXIV.) 
Only the head of this embryo is represented, as 
the general state of development is about the same 
as in the preceding stage. 
The chief object in making the figure is to show 
the five gill clefts (g’-*). The fifth cleft, though 
small and probably not open to the exterior, is 
quite distinct in this embryo. The writer would 
feel more doubt of its being a true, though rudi- 
