1909. } POSTCAVAL VEIN IN MAMMALS. 511 
rather regard as a mere variation of an individual character. In 
the European Hedgehog I found that the renal veins were quite 
or very nearly symmetrical. It ought also to be stated that the 
posteaval vein lay posteriorly to the right of the aorta, as is the 
normal position in EKutherian Mammals. In a second specimen 
there was a divided postcaval as in Centetes; but I am unable to 
give details. 
In Centetes ecaudatus (see text-fig. 136 A, p. 508) I found the 
conditions characteristic of the Armadillos somewhat exaggerated. 
In a specimen of that Insectivore which I dissected some two 
years ago the postcaval was formed of two parallel vessels from 
a point slightly in front of the influx of both renal veins anteriorly. 
A larger region was therefore double that in Dasypus &e. The 
renal veins were single on both sides, the complication of these 
veins so often seen among the Armadillos being in Centetes quite 
undeveloped. The two divisions of the posteaval after the influx 
of the renal veins were separated somewhat widely as in Dasypus 
&e.; but this separation was not complete. For two veins 
bridged over the intervening space, as is shown in the figure 
(text-fig. 136A). The first of these was slightly behind the 
left renal, and each slender vein joined its fellow to form a short 
backwardly running median vein. Further back still, just in 
front of the iliac veins, there was another small median vein 
formed by the union of two similar branches. The posterior 
mesenteric artery emerges from the aorta in front of the anterior 
bridge. The conditions thus shown are not seen in the Armadillos. 
But they exactly correspond (in that there are two bridges between 
the right and left postcavee) to a stage in the development of these 
veins in Hehidna aie by Hochstetter *, whose figures have 
been copied by McC ‘lure + 
Such anastomoses nilso% occur in Didelphys as stated by McClure. 
It is perhaps, therefore, possible to say that the presence of such 
anastomoses uniting the postrenal section of the postcaval, which 
is, as I presume, formed at least more anteriorly by the two sub- 
cardinals of the embryo, is an archaic character; for during the 
development of Didelphys such anastomoses occur of which there 
are in that Marsupial, as already stated, frequent traces in the 
adult in various positions and of differing degree, many of which 
have been deseribed and figured by McClure in his paper to which 
reference has been made so often in the present communication, 
But without actual developmental facts relating to Centetes it is 
of course dangerous to press any such comparisons very far. For 
it is equally possible that the anastomoses in question are a 
remnant of the cardinal collateral system. In Centetes, as in 
Erinaceus and other primitive mammals, each renal vein received 
from the hinder region of the body two veins which course along 
the ureter; one of these, as in other animals is the ovarian vein. 
* “Monotremen u. Marsupialier,” in Semon’s Zool. Forschungsreise in Australien, 
Bd. ii. Lief. iii. 
+ Amer. Journ. Anat. ii. 1903, p. 400, fig. x. 
