Charles F. W. McClure 201 
The following table shows the distribution of the above-mentioned 
types among 99 individuals; the postcava of the two remaining adult 
opossums, as mentioned above, not finding a place in the classification. 
TYPE 3 io) TOTAL 
TV DE RRs. cas arate ciskousl tt arSteis Sieisieesns aa ove 11 18 29 
AUS ViPV Gs Mee tarsy ches eye lio ierehigpslavieue of @gelomereneitey one 9 18 27 
Type III 
OD cs Bret ois OR ROME RCT oR NCO POE 3 5 8 
Fo ake ote de re one ese ee 9 15 oan 
CFR en rel cary cet eit oes 2 2 9 11 
IDX FT Beale Geen ee ee ae aE ee RE Ie EOE 34 65 99 
THE DEVELOPMENT OF THE THREE TYPES OF POSTCAVAL VARIATIONS 
Wuicuo NorRMALLY Occur IN DIDELPHYS MARSUPIALIS. 
Considering the uniform manner in which the three types of post- 
caval variations occur in the adult there can be little doubt that their 
development in the embryo is also a normal procedure, and that they 
are not abnormalities in the strict sense of the word. Also, the circum- 
stance that so many as 99 variations can be classed under so few as three 
types is certainly suggestive that there may be some common ground- 
type to which they can all be referred not only in the adult, but in the 
embryo as well. 
In the 8 mm. embryo of Didelphys, as stated above, the umbilical 
artery of each side, near its origin from the aorta, is encircled by a com- 
plete circumarterial venous ring. These venous rings (see Text Figs. 
10 and 13), as stated on a preceding page, are situated near the conflu- 
ence of the external and internal iliac veins and are extremely variable 
in their character, not only in different embryos but even upon opposite 
sides of the same individual. For example, the portion of the venous 
ring which encircles the umbilical artery ventrally may possess a caliber 
subequal with or greater in size than that which encircles it 
dorsally (Text Fig. 10 and Fig. 42, Plate III); or, the portion of the 
ring which encircles the artery ventrally may possess a smaller caliber 
than that which encircles it dorsally (Text Fig. 13 and Fig. 41, 
Plate IIT). 
This variation in caliber of the venous rings, as well as the relations 
the venous rings hold to the umbilical arteries in the 8 mm. embryos, 
is certainly suggestive of the conditions which characterize the three 
types of postcayal variations in the adult, in which a. correspond- 
ing variation in caliber is met with as regards the veitis which lie dorsal 
14 
