2 H. E. JORDAN 



with a brief description of the comparative histology of this 

 cord. Compared with the human cord it is very short, of con- 

 siderably lesser girth, and only slightly twisted. It has the 

 same light gray, pearly appearance, and feels of about the same 

 consistency. In transverse section it has an irregularly oval 

 shape (fig. 1), measuring 5 by 7 mm. Its three main blood- 

 vessels have an approximately equal caliber and thickness of 

 wall. It contains a large open allantoic duct and remnants of 

 the occluded yolk-stalk. The connective tissue contains many 

 arterioles, venules, and capillaries. Only one of the post-em- 

 bryonic human umbilical cords in my collection, a full-term 

 specimen, contains any blood-vessels besides the usual umbilical 

 arteries and vein. In this cord occurs a venule of considerable 

 size, lying near the surface and completely filled with red blood- 

 corpuscles. The human umbilical cord is typically non-vascular 

 except for an occasional capillary at the extreme proximal end. 

 None of my sections of these eight human cords contains any 

 vestige of the allantois. One cord contains a small, still patent 

 yolk-stalk; three contain a double, occluded yolk-stalk remnant. 

 One of the full-term cords of the pig also contains a double oc- 

 cluded yolk-stalk (figs. 1 and 3), the other only a single, small, 

 occluded remnant in only a few sections. In one of the human 

 cords the persistent, double, occluded yolk-stalk is enveloped 

 by a double layer of smooth muscle, an inner longitudinal and 

 an outer thinner circular layer. 



THE UMBILICAL ARTERIES AND VEIN 



In the cord of the pig the wall of the umbilical arteries only 

 contains circularly disposed smooth-muscle cells, more compact 

 centrally; the vein in one of the two specimens contains also 

 scattered, longitudinally placed cells beneath the intima. The 

 disposition of the muscle differs from that in the human cord 

 where the arteries and the vein contain, in addition to the chief 

 circular layer, also a thin internal longitudinal layer and scat- 

 tered bundles of longitudinally arranged cells externally. With 

 regard to the elastic tissue content also, the pig's cord differs 

 sharply from the human cord. The arteries of the latter lack 



