RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 35 



with those observed by McClure in Didelphis. We have here, 

 therefore, an undoubted instance in which, in an ungulate placen- 

 tal, the organization of the abdominal venous and lymphatic com- 

 plex dates back, phylogenetically, to the period of a common mam- 

 malian, or even amniote, vascular groundplan (text figure A, p. 

 41), forming the starting point from which all types of normal 

 and variant postcaval organization, monotreme, marsupial and 

 placental, radiate. This makes it possible, as in the ungulate 

 genus just mentioned, to find in otherwise valid placental forms, 

 as a constant and normal structural character, the postrenal 

 segment of the postcava as a single median preaortic vessel, which 

 receives, usually, both sex veins, and corresponds in every respect 

 to the characteristic marsupial venous type. This channel is the 

 final and permanent product of the fusion of the two embryonal 

 cardinal collateral veins, which, in the typical placental, either 

 do not develop at all, as distinct veins, or else, as in the cat, appear 

 during the ontogenseis only for a very short period, and are sub- 

 sequently entirely replaced by extraintimal lymphatic spaces 

 eventually uniting to form the extensive right and left ascending 

 lumbar lymphatic trunks. These vessels (75) are seen in the prep- 

 aration under discussion on each side, ventro-lateral of the aorta 

 in the space between the postcardinal veins (67 and 68} and the 

 cardinal collateral veins (74). These large lymphatic channels 

 have been developed through the fusion of numerous separate 

 extraintimal spaces surrounding and finally replacing some of the 

 elements of the earlier preaortic venous plexuses. A condensed 

 portion of the latter still persists in this embryo as the cardinal 

 collateral veins (74) above referred to. Eventually these also 

 are destined to undergo, in course of normal development, complete 

 extraintimal replacement by lymphatic channels. These latter 

 thus come to occupy in the typical placental secondarily the ter- 

 ritory which in monotremes, marsupials ajid atypical placentalia 

 (Tragulus) is filled by the postrenal preaortic postcava (cardinal 

 collateral). 



It is interesting to note in this connection that a detailed exami- 

 nation of over 900 adult cats failed to reveal a single instance in 

 which the postrenal segment of the postcava was formed through 



