1909.] POSTCAVAL VEIN IN MAMMALS. 525 
little remarkable. The Marsupials seem to be alone among the 
non-Monotreme Mammals in possessing invariably two pairs of 
spermatic (or ovarian—for the sex of the individual does not affect 
these veins) veins, one pair flowing into the posteaval direct and 
the other pair, often connected on its way with the former, open- 
ing into the renal veins. At least, it may safely be said that this 
is Sane the prevalent arrangement. And, furthermore, this 
group is also to be remarked upon as characterised by the symmetry 
of the posterior spermatic veins, though here the rule is not so 
universal. In other animals there would appear to be a persistence 
of either one pair of these veins or of the other; while some 
instances could almost seem to prove the retention of the left 
posterior spermatic vein and of the right anterior spermatic vein. 
The Dasypodide are apparently a group in which the posterior 
spermatic veins are beginning to vanish. Both pairs occur in 
Tatusia kapplert and only the anterior pair in some other forms. 
The details I have already given and need not recapitulate here. 
The Insectivora are a group which are in the same condition as 
the Edentata. In Centetes, for example, there is no posterior 
spermatic vein, while there is such a pair of veins in Hrinaceus 
europeus (at least occasionally). The higher Kutherian mammals, 
so far as they are known, never appear to possess both pairs of 
spermatic veins. More usually, as I hope to show shortly, the 
anterior spermatic vein persists on the left side and the posterior 
on the right. There are, however, a few forms in which it is the 
posterior pair only which remain, there being apparently in those 
forms no trace of the anterior pair. Thus in 7ragulus and Sciurus 
it seems to be obviously the posterior pair which are the only 
spermatic veins. Furthermore, in those Carnivora (at any rate 
Letonyx and Meltivora) which occasionally present us with a 
divided posteaval vein, the posterior spermatic veins persist on 
both sides, are symmetrical, and are the only spermatic veins. But 
in examples of Letonyx where the postcaval is single and in other 
Carnivora where no such duplication has been recorded, the 
conditions are what may be called the typical Eutherian 
conditions, 7.¢., the asymmetrical retention of veins. 
This asymmetrical retention of veins seems to be connected with 
the formation of the renal and postrenal sections of the postcaval 
vein. As seems to be now fairly certain from the investigations 
of Hochstetter *, Lewis t, McClure {, and of Soulié and Bonne §, 
already referred to, that region of the postcaval in the higher 
mammals is developed from the right-hand subcardinal (or 
perhaps collateral cardinal) and right postcardinal only, the left 
disappearing. The spermatic veins, therefore, on the left side 
obviously lose their connection with the postcaval through the 
disappearance of the intermediate veins. When the development 
is symmetrical, as in Z’ragulus, Mellivora, &., the genital veins 
* Toe. cit. pe 
+ “The Development of the Vena Cava Inferior,’ Amer. Journ. Anat. i. 1902, 
p. 229. t Loe. cit. § Loc. cit. 
