182 Venous System of Didelphys Marsupialis (1) 
Correlated with the atrophy of the mesonephroi, the cardinal collateral 
veins, or veins which are derived from them, assume the function of the 
posteardinals in returning the blood from the hind limbs and pelvic re- 
gion to the root of the postcava (pars subcardinalis) ; and, after fusing 
ventral to the aorta, constitute the greater portion of the stem of the 
posteava which is developed caudad of the original cross anastomosis be- 
tween the subcardinals. From a physiological standpoint the cardinal 
collateral veins of Didelphys may be said to correspond to that portion 
of the postcardinal in the cat and rabbit which is formed on the medial 
side of the permanent kidney and ureter, respectively. 
The Cardinal Collateral Veins——The cardinal collateral veins, as rep- 
resented in the reconstructions (Text Figs. 10 and 13) and in section 
(Figs. 40 and 41, Plate IIT) constitute an extremely complicated system 
of vessels which, in the 8 mm. embryo, are so irregular in character that 
it is difficult, at this stage,’to assign to them any definite ground-plan 
arrangement which may be regarded as characteristic of these veins in 
general. 
In some of the 8 mm. embryos examined the cardinal collateral veins 
appear to be present, for the most part on one side (Text Fig. 10), while 
in others they approach a bilateral arrangement as represented by Text 
Fig. 13. They may anastomose in front with the postcardinals (Text 
Fig. 10, left side) or, as is usually the case, with the root of the posteava 
as in Text Fig. 18. They may also extend caudad, on each side, parallel 
to the postcardinals either as single vessels or as a network of vessels 
which spread out in the space ventral to the aorta as in Text Fig. 13. 
The cardinal collateral veins often anastomose with each other in the 
median line ventral to the aorta (Text Fig. 10); they may also form 
frequent anastomoses with the postcardinal and posterior revehent (sub- 
cardinal) veins and, on being traced caudad, appear, in some cases to be 
directly continuous with that portion of the circumarterial venous ring 
which encircles the umbilical artery ventrally (Text Fig. 13, left side). 
In a few cases the cardinal collateral veins could be traced for a short 
distance caudad of the circumarterial venous rings where they appeared 
to terminate in capillary vessels (Text Fig. 13). 
The question as to origin of the cardinal collateral veins is difficult of 
solution and with the material at hand impossible to determine definitely. 
They do not, however, appear to be formed through a longitudinal anas- 
tomosis between the dorsal somatic branches of the posteardinals, but 
rather through a longitudinal anastomosis between the cross connections 
which exist between the post and subcardinal veins. 
Having considered the postcardinal, cardinal collateral and posterior 
