CHANGES IN FETAL VESSELS OF THE LIVER 501 



The conditions as found in this dog are quite different from 

 those found in the hypogastric arteries of a lamb three and one- 

 half weeks old a section of which is represented in figure 17. 

 In this case there are portions of the vessel in which the lumen 

 is completely surrounded by a thin layer of fibrous tissue (a) 

 which varies considerable in thickness but which lies internal 

 to the elastica interna (6). Although the endothelium is absent 

 in places the nmsculature is well-preserved where' it is not invaded 

 by the connective tissue, and contains many elastic fibers (c). 

 In the more distal portion the vascular f brous connective tissue 

 plugs the whole lumen of the vessel and serial sections toward 

 the tip show that it did not enter from without through the free 

 extremity. The intima is preserved in some places and the 

 connective tissue then lies between it and the elastica interna. 

 Where the intima is absent it lies in long strands in the lumen. 

 The connective tissue internal to the elastica ;s directly con- 

 tinuous in some places with that external to it which lies between 

 the muscle bundles of the media. Near the extremity of the 

 vessel the inner layers of the media are encroached upon and 

 entirely replaced by the vascular connective tissue which plugs 

 the entire lumen. Hence, I am inclined to believe that the proc- 

 ess of obliteration in this lamb is entirely comparable to and 

 represents a later stage than those found in the hypogastric 

 arteries of the dog as represented in figures 13 and 14. 



From these things it is evident that obliteration of the hypo- 

 gastric arteries in the dog and sheep may take place in at least 

 two ways. In one case the degenerating musculature is replaced 

 by fil)rous tissue which encroaches upon it and upon the lumen 

 from the periphery toward the center and in the other the lumen 

 is encroached upon and the surrounding musculature displaced 

 by connective tissue arising from within after degeneration and 

 desquamation of the intima and proliferation of vessels from the 

 vasa vasoi'um. It is evident of course that whenever this trans- 

 formation takes i)lace from without, i.e., from the periphery, 

 the lumen and its bounding intima as well as the inner layer 

 of the UKMlia, may be preserved very much longer than in the 



