SCHULTE, SEI WHALE. 469 



by the distal postcava finding its way to the heart by two paths, that of the postcava itself, 

 or through the intraspinal veins to the posterior thoracic veins and so through the brachio- 

 cephalicse to the precava. In consequence the subhepatic cava may be reduced in size, in 

 this foetus to but a very moderate degree, and this reduction has been cited by Beddard ' as evi- 

 dence of edentate affinities on the part of the Cetacea. Such cavo-spinal communications are 

 of wide distribution among the mammalia and are a normal feature of the more extensive venous 

 system of the embryo. It is therefore difficult to evaluate their taxonomic importance, even 

 when highly developed, and in view of the variability of the venous system their character ought 

 to be determined by numerous observations upon the forms compared. Morphologically they 

 must be regarded as phyletically primitive characters for the intraspinal path is highly organ- 

 ized and of large size in reptiles and not as specializations, except to the degree that the 

 selection and hypertrophy of a widely distributed character, if proved to be constant and uniform 

 in a species or order, may have this value. A venous peculiarity is a criterion which must be 

 used with extreme caution, and even when of considerable magnitude is not always a proof of 

 genetic relationship. In Marsupials and in Tragulus the postcava has in general the preaortic 

 position, yet in some Marsupials, departures from this type are recorded. In Petauroides 

 volans the postcava lies to the right and dorsal to the aorta (Hochstetter, Schulte) ; in Pseudo- 

 chirus it sometimes does so, sometimes is preaortic; in Didelphys marsupialis it may as 

 a variant have a left dorso-latreal relation to the aorta (McClure). 



The region of the subcardinal anastomosis has here the form of a confluence of the two 

 renal veins with the postcava. The renal veins emerge from the ventral surfaces of the kidneys 

 near their upper poles and ascend in a curved course to join the cava, the distal trunk of which 

 but little exceeds either of the renal veins in size. The left vein enters at a slightly lower level, 

 turning its proximal segment almost into the line with the cava as it does so. Here the veins 

 are demarcated for a short distance by a sulcus which prolongs their angle of union. The 

 left renal vein receives the left adrenal and the left phrenic veins as tributaries. The right 

 is joined by the phrenic of its side. 



The cava continues distad only a short distance as a single trunk. In the greater part of 

 the lumbar region it is double, the vein of the right side slightly preponderating in size. The 

 junction of the left postcava with the right is effected dorsal to the aorta by numerous oblique 

 branches of anastomosis. This condition is found in Monotremes, a resemblance which may 

 serve to illuminate that subsisting between Cetacea and Edentates (Xenarthra) in the cavo- 

 vertebral anastomosis. These communications ascend obliquely from left to right, and are 

 of considerable relative size. In the region rostrad of the hypogastric arteries nine were 

 present. By them, as may be inferred from their direction, blood is conveyed from the left to the 

 right postcava. The left vessel is thus depleted and the last of the oblique vessels connects 

 its termination with the right cava. Between these supracardinal anastomotic vessels, the 

 dorsal segmental branches of the aorta pass to their distribution. The whole arrangement, 

 apart from the size of the vessels, is very similar to that of Ornithorhynchus.' 1 



1 Beddard, F. E. Mammalia. Cambridge Nat. Hist., Vol. X, 1902, p. 120. Cf. Gregory, \V. K. Orders of Mammals. Bull. 

 Am. Nat. Hist., Vol. XXVII, 1910, p. 414. 



2 Hochstetter, F. Beitrage zur Entwickelungsgeschichte des Venensystems der Amnioten. 3. Sauger. Morph. Jahrb., Bd. XX, 

 1893. Schulte, H. von W. The range of variations in Monotremes and Australian Masupials. Proc. Am. Ass. Anat., 21st Sess. Anat. 

 Rec., Vol. 1, 1907. Schulte, H. von W., and Tilney, F. A note on the organization of the venous return with especial reference to 

 the iliac veins. Anat. Rec., Vol. 3, 1909. 



