The cardinal veins are comparatively late developments. There is at first one single primitive vein continuous in front with the heart and underlying the alimentary canal through its præanal and postanal sections. This vein is shewn in section in Pl. 11, fig. 8, V. It may be called either the subintestinal or splanchnic vein. At the cloaca, where the gut enlarges and comes in contact with the skin, this vein is compelled to bifurcate (Pl. 18, fig. 6,d, v.cau.), and usually the two branches into which it divides are unequal in size. The two branches meet again behind the cloaca and take their course ventral to the postanal section of the gut, and terminate close to the end of the tail, Pl. 18, fig. 6,c, v.cau. In the tail they form what is usually known as the caudal vein. The venous system of Scyllium or Pristiurus, during the early parts of stage K, presents the simple constitution just described.
Before proceeding to describe the subsequent changes which take place in it, it appears to me worth pointing out the remarkable resemblance which the vascular system of an Elasmobranch presents at this stage to that of an ordinary Annelid and Amphioxus. It consists, as does the circulatory system, in Annelids, of a neural vessel (the aorta) and an intestinal vessel, the blood flowing backwards in the latter and forwards in the former. The two in Elasmobranchii communicate posteriorly by a capillary system, and in front by the arterial arches, connected like the similar vessels in Annelids with the branchiæ. Striking as is this resemblance, there is a still closer resemblance between the circulation of the Scyllium embryo at stage K and that of Amphioxus. The two systems are in fact identical except in very small details. The subintestinal vessel, absent or only represented by the caudal vein and in part by the ductus venosus in higher Vertebrates and adult Fish, forms the main and only posterior venous trunk of Amphioxus and the embryo Scyllium. The only noteworthy point of difference between Amphioxus and the embryo Scyllium is the presence of a portal circulation in the former, absent at this stage in the latter; but even this is acquired in Scyllium before the close of stage K, and does not therefore represent a real difference between the two types.
The cardinal veins make their appearance before the close of stage K, and very soon unite behind with the unpaired section of the caudal vein (Pl. 11, fig. 9b, p.cav. and v.). On this junction being effected retrogressive changes take place in the original subintestinal vessel. It breaks up in front into a number of smaller vessels; the lesser of the two branches connecting it round the cloaca with the caudal vein first vanishes (Pl. 11, fig. 9a, v), and then the larger; and the two cardinals are left as the sole forward continuations of the caudal vein. This latter then becomes prolonged forwards, and the two posterior cardinals open into it some little distance in front of the hind end of the kidneys. By these changes and by the disappearance of the postanal section of the gut the caudal vein is made to appear as a superintestinal and not a subintestinal vessel, and as the direct posterior continuation of the cardinal veins. Embryology proves however that the caudal vein is a true subintestinal vessel[320], and that its connection with the cardinals is entirely secondary.
The invariably late appearance of the cardinal veins in the embryo and their absence in Amphioxus leads me to regard them as additions to the circulatory system which appeared in the Vertebrata themselves, and were not inherited from their ancestors. It would no doubt be easy to point to vessels in existing Annelids which might be regarded as their equivalent, but to do so would be in my opinion to follow an entirely false morphological scent.
The circulation of the yolk-sack.
The observations recorded on this subject are so far as I am acquainted with them very imperfect, and in most cases the arteries and veins appear to have been transposed.
Professor Wyman[321], however, gives a short description of the circulation in Raja Batis, in which he rightly identifies the arteries, though he regards the arterial ring which surrounds the vascular area as equivalent to the venous sinus terminalis of the Bird.
The general features of the circulation are clearly portrayed in the somewhat diagrammatic figures on Pl. 9, in which the arteries are represented red, and the veins blue[322].
I shall follow the figures on this plate in my descriptions.
Fig. 1 represents my earliest stage of the circulation of the yolk-sack. At this stage there is visible a single aortic trunk passing forwards from the embryo and dividing into two branches. No venous trunk could be detected with the simple microscope, but probably venous channels were present in the thickened edge of the blastoderm.