NO. II DIGESTIVE CANAL OF AMERICAN ALLIGATOR REESE 5 



at the basal ends of the ceUs, though an occasional nucleus may be 

 seen near the center of the layer. The mesoderm that extends 

 ventrad from the mesentery, on each side of the entoderm just 

 described, consists of a thick layer of compactly arranged cells. The 

 ventral end of the entodermal wall is fused with the wall of a small 

 cavity, !i, wdiich may be traced several sections cephalad to this plane. 

 This cavity is a part of the system of hollow liver trabeculae seen as 

 a group of irregular masses of cells ventrad to the enteron at the 

 opening of the anterior intestinal portal. The large blood vessel, bv, 

 is the meatus venosus. 



Figure 41 is just four sections caudad to the preceding. It passes 

 through the anterior intestinal portal, aip. The medial liver trabecula 

 into which the enteron was seen to open, in the preceding figure, now 

 opens ventrally to the yolk-sac as the anterior intestinal portal. A 

 few liver trabeculae are to be seen on either side of the portal, but 

 they show no lumena, and may be traced through only a few sections. 

 The extent of this uninclosed region, the midgut, is very difficult to 

 determine with accuracy, but, at this stage, it comprises about one- 

 half of all the sections of the series. The difficulty is due partly to 

 the unavoidable tearing of the tissues in removing the embryo from 

 the yolk-sac, and partly to the indefiniteness of the posterior intestinal 

 portal, where the walls of the enteron are very thin. As seen in 

 figure 41 the location of the anterior intestinal portal is very distinct. 



A short distance caudad to the anterior intestinal portal there is 

 constricted off from the roof of the midgut a narrow diverticulum, 

 figure 4J, i, the meaning of which is not apparent ; it extends through 

 only ten to fifteen sections, tapering caudad till it disappears. The 

 region of the hindgut, at this stage, is about one-fifth of the entire 

 length of the embryo. Its anterior portion is wide and, as has been 

 said, rather indefinite in outline. 



Figure 4K represents a typical section through the midgut region 

 of an embryo of about the age of the one from which the preceding 

 figures were drawn. This and the following figures of this stage 

 were drawn from an embryo in which the posterior region was in 

 better condition than in the embryo from which the other figures 

 of the stage were taken. The mesentery, nis, is here of considerable 

 length and continues around the yolk in a layer of diminishing thick- 

 ness. The epithelium of this region of the enteron consists of a 

 single layer of fairly regular cells, which are columnar in the dorsal 

 region, just beneath the mesentery, and cuboidal or even flattened in 

 regions more distant from the median plane. 



Figure 4L, through the region of the hindgut, shows at i the com- 

 pletely inclosed intestine ; it is a comparatively narrow tube, lined 



