UMBILICAL CORD OF THE PIG 7 



In the area between the lower umbilical artery on the left and the 

 vein on the right, the connective tissue is in part of the type 

 characteristic of the human cord; that is, it is typical mucous 

 connective tissue, but with an occasional capillary. In this 

 region also occur small bundles of collagen fibrils. The area 

 around the yolk-stalk contains vascular connective tissue of 

 an intermediate type, with occasional collagen fibers (fig. 5). 

 In the narrow space between the two umbilical arteries, extending 

 to a point below the allantoic duct, there occur several blood- 

 islands (figs. 6 and 7). About midway between the central 

 umbilical blood-vessels and the periphery occur numerous arte- 

 rioles and venules. Occasionally these are arranged in pairs 

 (fig. 5). These vessels terminate in capillaries. The arterioles 

 are enveloped by a thin layer of smooth muscle; the wall of the 

 venule only consists of endothelium resting upon the slightly 

 more condensed enveloping connective tissue. 



Certain of these smaller blood-vessels can be traced into con- 

 nection with the umbilical arteries and the vein at their proximal 

 (fetal) end. This is true both in the case of the younger cords 

 (of embryos from 9 to 21 mm.) and in those near full term. In 

 the 21-mm. fetus branches from the proximal end of the umbilical 

 vein can be seen entering the body wall as well as the connective 

 tissue of the cord. It may be confidently assumed that all of 

 the blood-vessels, including the numerous capillaries, connect with 

 vessels which ultimately connect with the main umbilical vessels 

 proximally. But not all of these vessels are properly interpreted 

 as vasa vasorum. Undoubtedly many function thus, as is indi- 

 cated by the numerous capillaries directed towards the walls of 

 the umbilical arteries and vein; but others are equally certainly 

 nutrient vessels for the allantoic duct, the ectodermal covering, 

 and the general connective tissue. Nor can the assumption be 

 properly made that all of these vessels arise by sprouting from 

 originally direct umbilical branches. If the capillaries grew ex- 

 clusively by sprouting, their tips should show mitotic figures. 

 Mitotic figures are practically absent in these capillary terminals. 

 On the contrary, these tips seem to fuse with the general con- 

 nective tissue, the cells of which become hollowed out, arrange 



