Charles F. W. McClure 21 
~2 
of the embryos, maintains essentially the same relations as in the preced- 
ing stage and, after tunnelling the liver, also opens into the posteava inde- 
pendently of the umbilical veins. In the 10.5 mm., as well as in all of 
the older pouch young, the hepatic continuation of the omphalomesen- 
teric vein, instead of forming a continuous channel through the liver to 
the postcava, is broken up by the hepatic sinusoids so that the omphalo- 
mesenteric vein now functions as an advehent vein of the liver, or, in 
other words, has become transformed into the portal vein (Text Fig. 26). 
From the above description of the hepatic circulation in Didelphys it 
is evident that it differs widely from that met with in monotremes or 
in any of the higher mammals thus far described. Although in mono- 
tremes the ductus venosus Aranzii differs from that of the higher mam- 
mals in that it does not form the direct continuation of the left umbilical 
vein, it is nevertheless formed through a union of the portal and umbili- 
cal veins (Hochstetter, 96). In none of the embryos of Didelphys studied 
by the writer was a ductus venosus Aranzii formed as in monotremes or 
placental mammals, since in every instance noted both the umbilical and 
omphalomesenteric veins passed through the liver without at any time, 
except by sinusoids, anastomosing with each other. It, therefore, ap- 
pears questionable to the writer whether one can rightly speak of a ductus 
venosus in Didelphys that would correspond to that in monotremes and 
placental mammals, and for this reason this term has been omitted in 
the preceding description of the hepatic circulation. 
The nearest approach to the conditions deseribed above for the om- 
phalomesenteric and umbilical veins of Didelphys appear to be present 
in birds as described by Hochstetter (03, Fig. 156, page 136). Here the 
omphalomesenteric and left umbilical veins open into the proximal end 
of the postcava without previously anastomosing with each other within 
the liver, except by sinusoids. 
tESUME AND GENERAL CONSIDERATIONS. 
For convenience of description, the posteava of the adult Didelphys 
was described in Part I (page 386) as consisting of the following sub- 
clivisions : 
A prehepatic division which includes that portion of the vein which 
extends between the right auricle and the most cranial of the hepatic 
veins; an hepatic division which is embedded in the liver and which in- 
cludes that portion of the posteava into which the hepatic veins open; 
a renal division which includes that portion of the posteava which lies 
between the most caudal of the hepatic veins and a point just behind the 
most caudal of the two renal veins, and a postrenal division which con- 
15 
