UMBILICAL CORD OF EMBRYO OF 17 MM. 239 



mesenchyma in which there is no very clear evidence of specialization. Between 

 the Wolffian bodies is suspended the large intestine. It has a small canal formed 

 by entoderm and very thick mesodermic walls. Attached to the ventral side of 

 the body- wall of the embryo is the allantois, All, the cavity of which is quite 

 large, somewhat irregular in shape, and lined by a cuboidal epithelium, a portion 

 of the entoderm. By following through the sections it can be seen that the allan- 

 tois and large intestine join at the cloaca. The entodermal allantois is sur- 

 rounded by mesenchyma, which is very much looser in texture than that of the 

 intestine proper. On either side of the allantois is a projecting lobe of tissue in 

 which the umbilical artery, A. um, is lodged. The two arteries pass upward to 

 the umbilicus, then outward to the placenta. Downward they continue to the 

 level of the cloaca, there pass to the dorsal side of the embryo, and unite with the 

 end of the median dorsal aorta. 



Frontal Section of the Umbilical Cord of Embryo of 17 mm. 



Since the umbilical cord projects from the abdomen, we get in frontal series 

 of the embryo sections of the umbilical cord which are more or less nearly trans- 

 verse. Such sections are instructive (Fig. 139). In the illustration we see that 

 the umbilical cord is formed chiefly by mesenchyma, mes. It contains four 

 blood-vessels; two umbilical veins, of which the left, U. V. S, is enlarged, while 

 the right, U. V. D, is diminished in size. The arteries. Art, are almost symmetri- 

 cal in position and alike in size. They are much smaller than the veins, but the 

 mesenchymal tissue about them is somewhat condensed, so that they are provided 

 with an imperfectly differentiated muscular coat. The body-cavity of the em- 

 bryo is prolonged into the cord, forming a central space, Coe, in which are lodged 

 the loops of the intestine and the prolongation of the allantois. The intestine is 

 cut twice, the section on the left passing through the ileum, and on the right 

 through the jejunum, which is much larger than the ileum, having both a smaller 

 entodermal portion and a thicker mesenchymal part. The two segments of the 

 intestine are joined together, and in the part between them are two blood-vessels, 

 one the vitelline artery, A . vi, and the other the vitelline vein. The mesenchyma 

 of the intestine and of the bit of mesentery between them consists of very crowded 

 cells, so that the tissue appears darkly stained, and offers in this respect a strik- 

 ing contrast to the allantois. All, the mesenchymal walls of which contain only 

 loosely scattered cells. The entoderm of the allantois is quite thin. The exter- 

 nal surfaces of the intestines and of the allantois, and the outer surface of the 

 coelom, are all lined by a distinct layer of mesothelium. The ectoderm, Ec, is 

 thin, and consists, for the most part, of a single layer of cells, although the forma- 

 tion of a second outer layer seems to be beginning. 



