34 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



topographical field involved. The strands of the sympathetic (l) 

 are supra-aortic . On each side of the aorta (7) are the large and sj^m- 

 metrically developed right and left postcardinal veins {67 and 68), 

 with the ascending trunks of the ilio-lumbar arteries (A.ilio-lumb. 

 transv. ant.) (not labelled in the figure) applied to their ventro- 

 lateral circumference. The subaortic area shows the cross-sections 

 of four symmetrical vascular channels, two venous and two lym- 

 phatic. Immediately vential to the aorta, and closely applied to 

 its ventral wall and to each other, are two longitudinal parallel 

 axial veins which are connected at intervals by a few^ short trans- 

 verse anastomoses. These vessels are the temporary and very 

 evanescent homologues in the placental embryo of the channels 

 which McClure'"' has described as the "cardinal collateral trunks" 

 in the embryo of Didelphis marsupialis, and from which he has 

 traced the development of the preaortic postcava characteristic 

 of the Marsupalia. These vessels are derivatives from the earlier 

 preaortic cardinal-subcardinal venous plexus below the cross- 

 anastomosis, but differentiate in Marsupials along separate and 

 distinct axial lines. They are destined, as are the corresponding 

 portions of the subcardinals proper, to be entirely replaced in the 

 typical placental development by the chain of preaortic lymph 

 channels and nodes, but are capable, in aberrant types among the 

 placontalia, of yielding, by further and continuous development, 

 a type of pi-eaortic postcava which in every respect corresponds 

 to that encountered in Marsupials. McClure^' has described this 

 condition in Tragulus, and his observation has been confirmed 

 in a number of dissections by Beddard and others. The fortunate 

 ac(iuisition recently of a series of Tragulus embryos, through the 

 kindness of the officials of the Smithsonian Institution, has enabled 

 my associate Tilney to trace, in a pubhcation now in preparation 

 for Ihc i)ross, the development of the venous and lymphatic 

 systems in this aberrant ungulate in theirmutual interdependence, 

 and to show the correspondence of the venous genetic processes 



»• C. F. W. McClurc: "The Anatomy and Development of the Postcava in 

 Didelphis marsupialis." Am. Jour. Anal., vol. v, 1906. 



»' C. F. W. .McClure: "The Postcava of an Adult Indian Chevrotain {Tragulus 

 meminnn, Erxlehen). Anal. Am., Band, xxix no. 13 and 14, 1906, pp. 375-377. 



