31 6 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 {he) and the posterior cardinal veins 

 {pc) are very small in this region, as might be 

 expected. 



STAGE XV 

 Figure i8 (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- 



