526 ON THE POSTCAVAL VEIN IN MAMMALS. [Apr. 27, 
are also necessarily symmetrical. Here are both embryonic veins 
preserved with their branches, naturally equivalent on both 
sides of the body. But in the other cases, the spermatic vein 
of the left side only retains its connection with the cardinal 
(doubtless through the rich anastomoses which occur in the 
embryo), and follows the course of that vein anteriorly, opening 
into the renal in the left side. This point of view is confirmed 
by a consideration of those cases (of which I have described two 
in the present communication) where the left subcardinal or 
cardinal collateral, as the case may be, is converted into the 
postrenal section of the postcaval ; here the conditions of the 
spermatic veins are precisely reversed, as would be expected on 
this view. There remain, however, some instances which can 
perhaps be disposed of on this hypothesis. There is, for example, 
the single postcaval vein in the genera Seiurus and Zapus. As 
to the latter, however, it is quite possible. that its median position 
is significant. It may be that the two veins of the embryo which 
are connected with the formation of this part of the postcaval 
have both persisted and fused together, which would at once 
account for the median position and for the retention of both 
posterior spermatic veins. In the Squirrel, on the other hand, 
the postcaval postrenally 1 is distinctly developed to the right af 
the aorta as in most Mammals. Still we know nothing of the 
development of the veins in this mammal, and a study of the 
development might show that there was after all a fusion of right 
and left veins of the embryo. Besides these cases we have the 
peculiarly constant mode of connection of the spermatic veins 
with the postcaval in the Murine Rodents—in many of them 
at any rate, in Votiomys to take a definite example. Here we 
have a symmetry between the spermatic veins but of a different 
kind to that which occurs in other animals, save for a few 
exceptions. The left spermatic opens into the left renal, while 
the right spermatic is symmetrical with it and therefore does not 
open into the renal of its side but into the postcaval opposite to 
the point of entrance of the left renal. In these cases it appears 
to me that we may have the disappearance of both posterior 
genital veins and the retention of both anterior veins. This is, of 
course, in the absence of embryological data sheer surmise; but 
we already know of cases where this appears more definitely to 
be the case, such as a number of Armadillos and the Insectivore 
Centetes. I believe that the cases which have now been considered 
exhaust the variations known to exist In mammals. From an 
evolutionary point of view it seems likely that the existence of 
two pairs of spermatic veins is the more primitive state of affairs, 
since these veins are more numerous in the lower Amniota, and 
that the prevalent arrangement of these veins as one pair is the 
later state. This also (it will be observed) agrees with the mutual 
relations of the groups of mammals which are here dealt with. 
