A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 141 



ALLANTOIC DUCT. 



The allantoic duct is a very long, slender, hollow tube which proceeds from 

 the cephalic end of the cloaca and extends into the body-stalk. At its origin it 

 is funnel-shaped, and its lumen is distinct. It tapers rapidly as it enters the body- 

 stalk. For a few sections it becomes almost lost from view, owing to the indis- 

 tinctness of cell boundaries and scattered nuclei. Although continuity of the 

 allantoic cells can be made out, its lumen is lost. A few sections farther distally 

 the allantoic duct again becomes distinct and a trifle larger. Lying between the 

 two umbilical arteries, it follows the ventral bending of these vessels. Still lower 

 down the arteries fuse and then split apart again, thus forming an arterial fork. 

 The small allantoic duct passes in front of the fused part, and then, turning dor- 

 sally, passes through the above-described fork. Crossing the umbilical stalk 

 obliquely, it terminates in a small bulb, the allantoic vesicle, which is situated close 

 to the fused umbilical veins. 



CLOACAL MEMBRANE. 



A short distance from the end of the gut is a very slight outward bulging of 

 the ventral wall. This portion of the cloacal wall is in contact with the ectoderm 

 of the proctodeal invagination, and together these layers of epithelium form the 

 cloacal membrane (text-figs. 8 and 9). The entodermal portion is slightly thicker 

 than the ectodermic. 



CAUDAL INTESTINE. 



The portion of the gut beyond the cloacal membrane ends bluntly in the 

 extreme end of the tail, separated from the ectoderm by only a small amount of 

 mesenchymal tissue. This portion of the gut represents the post-anal or caudal 

 intestine (text-figs. 8 and 9). 



HISTOLOGICAL STRUCTURE OF THE DIGESTIVE TUBE. 



Histologically considered, the digestive tract may be described as an epithelial 

 tube surrounded by mesenchyma. Only the former shows signs of differentiation 

 as yet, the mesenchyma being everywhere of the same character. The epithelium 

 takes on widely different appearances in different regions. In general it may be 

 said that throughout the whole of the digestive tube, with the exception of the 

 lower end, the dorsal wall is much thinner than the ventral. The former is made 

 up of a single layer of cubical or flattened cells. On either side of the mid-dorsal 

 line the epithelium gradually becomes thicker and the nuclei more crowded. The 

 side-walls and floor of the pharynx show from two to three layers of nuclei. At 

 the places where the entodermal epithelium of the pharyngeal pouches comes in 

 contact with the ectoderm, it is thin and fused to the ectoderm. The membrane 

 closing the second gill-cleft on the left side has broken through to the outside; 

 this is undoubtedly a mechanical tear. The wall of the thyroid diverticulum 

 is not different from that of the floor of the pharynx, being composed of an epithe- 

 lium of two to three cell-layers. The epithelium of the ventral wall of the re- 

 mainder of the fore-gut is thicker, the change from the thin dorsal wall to the 



