NO. II DIGESTIVE CANAL OF AMERICAN ALLIGATOR — REESE 9 



the next reconstruction. The duodenum extends only a short dis- 

 tance candad to this point and then opens, aip, to the yolk-sac. 



The yolk-stalk, or unclosed region of the enteron, is still of con- 

 siderable extent, though its exact boundaries are not easy to deter- 

 mine. The distance between the anterior and posterior intestinal 

 portals is approximately shown in the figure under discussion. 



The hindgut is cylindrical in cross section and of about the same 

 diameter throughout, except for a slight enlargement in the cloacal 

 region. 



The post-anal gut is not shown here ; it will be described in con- 

 nection with the next reconstruction where it is figured. 



Figure 6 is a surface view in profile of an embryo of the next 

 stage to be studied. The manus and pes are here well developed, 

 and the general development of the embryo is in considerable ad- 

 vancement over the last stage studied. 



Figure 6a represents a reconstruction, from a series of transverse 

 sections, of the enteron of an embryo of about the age of the one 

 shown in figure 6. The outlines of the entire embryo, of the eye, e, 

 and of the anterior, aa, and posterior, pa, appendages are shown by 

 broken lines. Its position being coincident with that of the stomach, 

 liver, and pancreas, the anterior appendage can scarcely be seen. 

 The enteron, including one lung only, for the sake of simplicity, is 

 shaded solid black, while the liver and pancreas, with their ducts, 

 are outlines with unbroken lines. As in the preceding reconstruction 

 no attempt is made to show the gill clefts, and only the dorso-ventral 

 profile of the enteron is shown. Caudad to the pharynx, the enteron 

 being more or less cylindrical in section, this profile gives a good 

 idea of its shape, but in the pharyngeal region, where the lateral 

 diameter is so much greater than the dorso-ventral, the reconstruction 

 gives but a poor idea of the size of that part of the enteron. 



The widely-open mouth, in, leads, with no line of demarkation, 

 into the pharynx, ph, which is of irregular outline and, as has been 

 said, of much greater lateral than dorso-ventral diameter. 



The pharynx becomes gradually constricted to form the oesoph- 

 agus, oe, a very long and slender structure, which, as will be seen 

 in cross section, is, at this stage, solid for the greater part of its 

 length. As in the case of the pharynx, the lateral diameter of the 

 oesophagus is generally greater than the dorso-ventral diameter. 



From the floor of the caudal part of the pharynx is pushed out the 

 trachea, ta. In the reconstruction, especially in the anterior end, 

 the trachea appears several times the diameter of the oesophagus ; 

 this is due to the great thickness and indefiniteness of its walls rather 

 than to a greater diameter of its lumen. 



