190 Venous System of Didelphys Marsupialis (L) 
the right, the left, or on both sides of the aorta (Fig. 58, Plate V) ; a cir- 
cumstance which, we will see later, accounts for the variations in con- 
nection with the lumbar veins, as well as the variable manner in which 
the caudal end of the azygos may join the postcava in the adult (McClure, 
03, p. 382). One of these anastomoses invariably occurs slightly caudad 
of the renal veins and another at the junction of the postcava and the two 
posteardinal veins and between these two some other anastomoses are met 
with which are apparently more or less variable in character. 
Remarkable changes, as will be more fully described later on, have 
also taken place in connection with the cardinal collateral veins. These 
veins, at their cranial ends, anastomose with the root of the postcava 
(pars subeardinalis) and at their caudal ends with the postcardinals ; 
and, in correlation with the atrophy of the mesonephric divisions of the 
postcardinals (Urnierennabschnitt) have so increased in size that in most 
of these pouch young they now return to the postcava practically all of 
the blood collected by the tributaries of the external and internal iliac 
veins. The original mesonephric divisions of the postcardinals now 
function chiefiy as mesonephrie veins (Urnierenvenen) which return 
blood from the mesonephroi to the pars subcardinalis of the postcava. 
In the 17 mm. pouch young the arrangement of the venous system is 
essentially the same as in the adult, so far as the completion of the post- 
cava and the formation of the azygos system are concerned. 
The cardinal collateral veins now receive all of the blood collected by 
the tributaries of the external and internal iliac veins and constitute that 
portion of the unpaired postcava which les in the adult caudad of the 
spermatic veins. The mesonephroi, although more atrophied than in the 
14 and 15 mm. pouch young, are still functional and are connected with 
the body walls by means of extremely narrow mesenteries. ‘The meso- 
nephric divisions of the postcardinals have entirely ceased to be continu- 
ous vessels and are now represented by small veins which return the blood 
from the mesonephroi to the postcava. 
The changes, about to be described, that have taken place in connection 
with the azygos veins are without doubt correlated with the atrophy of 
the mesonephrie divisions of the postcardinal veins and the consequent 
completion of the postcava. 
In the 17 mm. pouch young the lumbar azygos vein now forms with 
the thoracic azygos of the left side a single, continuous vessel of consid- 
erable size which extends between the left precava and a point slightly 
caudad of the renal veins where, in the pouch young at hand, it joins 
the posteava on the right side of the aorta. Caudad of this point of 
junction with the posteava the lumbar azygos now ceases to be a continu- 
