DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 5 1 



embryo. The digestive tract is cut through its extreme posterior 

 end, in the region that may be .termed the cloaca (el), for into it at 

 this point the Wolffian ducts open (zvdo). -As the narrow cloacal 

 chamber is followed toward the tail, it becomes still smaller in diam- 

 eter, and the ventral depression or cleft seen in this figure gradually 

 becomes . deeper until its walls are continuous with the ectoderm 

 that covers the ventral projection 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 (be) and 

 the posterior cardinal veins { he) are very small in this region, as 

 might be expected. 



Stage XV 



Figure 18 (Plate XIX) 



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 1 '*). 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 rudimentary, gill cleft had 

 not Clarke (5) found a fifth pair of clefts in his material. The 

 nasal pit has advanced in development somewhat and shows the 

 beginning of the groove that connects it with the mouth. In this 

 figure the crescentic hyomandibular cleft shows its connection with 

 the groove between the mandibular and the hyoid folds. 



Stage XVI 



Figure 19 (Plate XIX) 



This embryo is only slightly more developed than the preceding. 

 Body flexure is so great that the forebrain and tail nearly touch. 

 Only the anterior three gill clefts are visible. The maxillary pro- 

 cess (mx) is longer and more narrow; the mandibular fold has not 

 changed appreciably. The nasal pit (n) is now connected by a dis- 

 tinct groove with the stomodaeum. The appendages have increased 

 in size, the posterior (pa) being the longer. The anterior appendage 

 (aa) is distinctly broadened to form the manus, while no sign of 

 the pes is to be seen at the extremity of the posterior appendage. 

 The heart (ht) is still very prominent. The stalk of the Umbilicus 

 (u), which is quite narrow, projects from the ventral wall in the 

 region between the heart and the hind legs. The tail is of consider- 

 able length and is closely coiled. 



