THORACIC DUCT DEVELOPMENT IN THE PIG 411 



In order to emphasize the principle involved in the extra-intimal 

 development of the mammalian lymphatics, Huntington selected 

 the thoracic duct as an example for the reason that the histoge- 

 netic processes which enter into its inception and completion are 

 clearly expressed and easily followed, and also because the duct 

 retains a more definite and constant position relative to sur- 

 rounding structures than perhaps any other lymph channel. He 

 observed that the continuity of the thoracic ducts is realized by 

 the confluence of a large number of spaces which have sprung from 

 the mesenchyme immediately in contact with decadent venous 

 channels. Their first appearance is as numerous intercellular 

 isolated fissures which then coalesce to form larger spaces, and 

 these in turn become confluent to produce the continuous vessels. 

 Lined with undifferentiated tissue cells at their beginning, they 

 gradually assume the flattened and delicate endothelium of the 

 adult lymphatic. 



McClure has described the same process of development in the 

 formation of the mesenteric lymphatics of the cat. 17 He showed 

 that at a certain period, a plexus of veins situated in the dorsal 

 mesentery becomes detached from the postcava and soon after 

 manifests signs of atrophy. With the aid of several clear micro- 

 photographs he further pointed out that the mesenteric lympha- 

 tics follow topographically, or better, appropriate these aban- 

 doned venous channels by a process of extra-intimal replace- 

 ment. The haemal endothelium collapses and large mesenchymal 

 spaces appear around it. In this connection he states: 



These lymph spaces which lie external to the intima of the veins gradu- 

 ally encroach upon the territory formerly occupied by the veins and 

 finally fill it completely; the result being that the original intima of the 

 vein, no longer serving in the capacity of lining a functional venous 

 channel, gradually degenerates and disappears. Traces of this intima 

 can often be observed, however, in older embryos, clinging to the walls 

 of the lymph channels within which a new lymphatic intima has been 

 established. 



That these pictures are real, and not artifacts induced by poor 

 preservation of tissue, is conclusively shown by the fact that they 



17 Charles F. W. McClure: 1910; loc. cit. 



