Charles F. W. McClure 391 
Type. | Q | Q Total. 
ype liccee fee i 18 29 
My eTap werd soe museca 9 18 27 
Mv per Tle. «<i center 
PARTE AES Sc ct Sean enee 3 5) 8 
By ene A. eee 9 15 24 ; 
Gre Gh. coger 2 9 11 
Rotate. ht: ee ae 34 65 99 
With the exception of eleven individuals which were about half-grown, 
the above observations were made upon adults. The variations observed 
in the half-grown opossums were in all respects similar to those of the 
adult, and I may state here that the Type of postcava which will be 
found in the adult is already indicated in the embryo at the time of its 
birth. Of the three main Types of variations the third easily predomi- 
nates, while there is not much difference between the first and second. 
The table, likewise, does not show any marked distribution of the Types 
among either sex, so I think it may be stated with certainty that no 
relation exists between sex or age and any particular Type of postcava. 
From a study of their development the writer is now able to account 
for these variations by showing that they are modifications of a “ ground 
plan ” arrangement which is common to the veins in the embryo. This 
question has already been dealt with in a preliminary paper, 02, and will 
be more fully treated in a subsequent paper on the development of the 
veins. 
THE VARIATIONS IN THE MODE OF ORIGIN OF THE POSTCAVA IN 
Didelphys Marsupialis. 
Type I—Includes those cases in which the Vv. iliace interne 
unite with the Vy. iliace externe to form the postcava, ventral 
to the Aa. iliace communes or ventral to the aorta. 
Twenty-nine examples of this Type were met with distributed among 
eleven males and eighteen females. 
See Figs. 1, 2, 3 and 4 (ventral views) and Fig. 5 (dorsal view), 
Plate I. 
The representatives of Type I more closely approach the conditions 
