42 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



The lymphatic channels replacing the subcardinal and cardinal 

 collateral venous trunks form the extensive system of the adult 

 ascending lumbar and preaortic lymphatic vessels and nodes 

 (2 f and 3' in figs. B and C). 



Of course it is quite apparent that the adult placental differ- 

 entiation occurs in the district of the post- and supracardinal 

 lines, with a strong predilection for the right supracardinal as 

 the main path of the postrenal segment of the adult postcava. 

 It is equally apparent that in correctly valuing the significance 

 of the departures from the normal type of placental postcava 

 all four of the available components, viz., right and left post- 

 cardinal and right and left supracardinal lines, must be taken 

 into account as potential factors in the development of the atyp- 

 ical placental postcava. The relation of the veins to the ureter 

 will then decide the question of the genetic derivation, as being 

 either the persistent postcardinal or supracardinal channel of either 

 the right or left sides, in the case of single trunks, or of both sides 

 in instances of double bilateral adult channels. 



Thus all the recorded cases of variant postcaval veins of the 

 cat, and of man, can be clearly interpreted on this basis, as has 

 been done by McClure, Darrach and myself in previous publi- 

 cations. 23 ' 24 ' 25 



Furthermore, the placental types in which a normally so-called 

 double postcaval vein occurs, as, e.g. in some of the aquatic 

 carnivores, some insectivores and edentates, are readily led back 

 to persistence of both right and left axial channels with absence 

 or reduction of the iliac anastomosis. Again the position of the 

 ureter in reference to the bilateral trunks will characterize each 

 of them as being either post- or supracardinal in derivation. 



In the marsupials McClure's researches already quoted, 31 

 show clearly that the members of this subclass depend upon the 

 continued development of the ventral preaortic venous path- 

 ways (2, 3} of the common vertebrate groundplan (fig. A) for 

 the evolution of their typical ventral preaortic postrenal seg- 

 ment of the postcava, with consequent reduction of the post- 

 cardinal line (.7) to the role of a sex vein terminal, and the complete 

 suppression of the typical placental supracardinal lines (4) in most 



