198 Venous System of Didelphys Marsupialis (L) 
During the migration of the kidneys the ureters also undergo certain 
changes in position. In the 8 mm. embryo they lie dorsal to the cardinal 
collateral and subeardinal veins along their entire extent and are not, at 
this period, so far as the writer can observe, encircled by venous loops. 
In the 14 and 15 mm. pouch young, on the other hand, at least in those 
cases in which the mesonephric divisions of the postcardinals have not 
yet atrophied, each ureter passes through a venous loop (Text Figs. 16 
and 17) which is formed laterally by the postcardinal and medially by 
the cardinal collateral vein. The relation of the ureters to the veins at 
this period of development thus resembles the conditions met with in 
some placental mammals (cat and rabbit) with the exception that among 
the latter the medial side of the loop lies dorsolateral instead of ventro- 
lateral to the aorta, as is the case in the opossum. 
The manner in which the ureters migrate from a position dorsal to 
the cardinal collateral ves to the position which they occupy in the 
14 and 15 mm. pouch young, ventral to these veins, is not clearly shown 
in the stages studied. J am inclined to believe, however, that the appar- 
ent discontinuity, mentioned above, which exists caudally between the 
posteardinal and cardinal collateral veins in the 10.5 and 11.5 mm. pouch 
young has been brought about by a ventral migration of the ureters. 
THE SPERMATIC VEINS. 
In the 8 mm. embryos of Didelphys, the blood from the anlages of the 
genital glands is collected by a number of tributaries (genital veins, 
Figs. 38 and 39, Plate III), which open into the derivatives of the sub- 
cardinal veins (pars subcardinalis of the postcava, the left anterior and 
posterior revehent veins). 
At a subsequent stage of development (14 and 15 mm. pouch young), 
with the retraction of the mesonephroi from the body walls, the genital 
veins return their blood to the posteava (pars subcardinalis) by means 
of a right and left mesonephric vein which opens into the postcava, 
caudad of the renal veins, at a point which corresponds to the junction 
of the postcava and the postcardinal veins (Figs. 59, Plate V). Since 
this junction corresponds to the level at which the spermatic veins open 
into the postcava in the adult, I am, therefore, convinced that these two 
mesonephric veins, together with their genital connections, are retained 
in the adult as the spermatic veins, although I have not been able to es- 
tablish definitely that such is the case. 
The spermatic veins in all of the adult opossums examined by the 
writer (101) were connected with the posteava caudad of a point mid- 
