CHANCiES IN FETAL VESSELS OF THE LIVER 495 



foriuecl tissue. Still farther centrally the regular, reduced lumen 

 which contains blood is surrounded by a well-preserved intima 

 and a better preserved though degenerating musculature (fig. 8) . 

 In the case of this dog then the peripheral portion of the lumen 

 of the vein became completely obliterated through connective 

 tissue formation while its musculature was degenerating and 

 being penetrated by numerous vessels arising from the vasa 

 vasorum, in spite of the fact that the vessel was to undergo 

 rapid degeneration and absorption. 



1/''. » -."^ 



\ 







Fig. G Central portions of umbilical vein of a dog 91 hours old, showing the 

 lumen obliterated by vascular connective tissue surrounded bj^ the degenerating 

 media. X275. 



The presence of such profound changes so soon yost natum 

 confirms the observation that embryonic regression of the um- 

 bilical vein sometimes begins before birth. In several instances 

 it was noticed, for example, that the cross-section of the media 

 of the umbilical vein lacked all the characteristics of plain muscle 

 and looked not inilike a syncytium. Such appearance could, 

 of course, be explained only by prenatal degenerative changes 

 or by the supposition that the musculature of the wall of the 

 vein sometimes fails to reach maturity during fetal Ufe — a 



