drained forward from the kidneys by a persistent postcardinal trunk on each side.

The efferent renal veins, the factors of the renal veins proper, show extreme variation in size, number and arrangement. They cover the greater part of the ventral surface of each kidney, lying more or less transversely across the organ. As they pass in towards the middle line of the body, they widen owing to the reception of innumerable, irregular tributaries, among which considerable anastomosis is shown. The efferent venal veins draining the narrow anterior region of the kidney usually show a more regularly parallel arrangement than those from the rest of the organ. At the inner border of the kidney, the efferent renals form by their confluence the renal veins, often converging in groups to do so. Most usually, the posterior half to two-thirds of the kidney is drained by a single, very stout renal vein which takes its origin near the posterior extremity of the organ. The renal veins from each side pass forward, often closely associated with the inner borders of the kidneys and receiving many efferent renal tributaries. Before the narrow, anterior third of each kidney is reached, the main renal veins converge towards the median plane of the body, finally uniting to form the stout, median postcaval vein. Very often, the anterior, converging region of each renal vein is joined by a slender vessel which passes directly inward from the kidney and is formed by the union of several efferent renal veins anterior to those draining the main vein. In some cases, several renal veins occur on one or both sides, but these always unite in various ways to produce a single main vessel before the postcaval vein is formed. The renal veins of opposite sides may be connected by transverse vessels. In the anterior third of the kidney, the postcardinal vein itself functions as a renal vein and receives a varying number of more or less parallel efferent renal tributaries. It is closely applied to the inner border of the kidney, but posteriorly it curves away from the latter to unite either with the postcaval vein or with the main renal vessels of the same side. Thus it appears that some of the blood drained into the postcardinals reaches the liver by way of the postcaval.

Blood from the gonads and fat-bodies eventually enters either the postcaval or postcardinal veins. The spermatic or ovarian veins are slender vessels, varying in number from three to six. The fat-body is usually drained by a single small vein which passes directly into the postcardinal vein, entering the postcardinal slightly beyond its posterior point of attachment to the kidney. This is sometimes joined by one or more of the veins from the anterior of the gonad, but it is equally as common to find all the ovarian or spermatic vessels entering the corresponding main renal vein or its tributaries, sometimes uniting in pairs before doing so. In the male frog, especially, several small connecting vessels may occur which link up the veins of individual gonads as well as those of opposite sides.

A striking degree of individual variation is encountered in the arrangement of the renal veins and their tributaries, six examples of which are figured in Plate 44.

In Figure I (a), the right kidney of a female frog is shown.

The numerous efferent renal veins and their finer tributaries are very irregularly arranged in the more posterior region of the kidney, but they are more or less parallel in the anterior portion, where they open into the postcardinal vein. The main renal vein is a large trunk, arising near the posterior margin of the kidney. It is closely attached to the inner border of the posterior third of the kidney, but beyond this point it forms a shallow loop towards the middle line of the body. Finally, it passes forward and inward to form the post-caval vein by union with the main renal vein from the opposite side. A more slender renal vein enters the postcaval vein directly, slightly behind the point of entrance of the posterior portion of the postcardinal. Four small veins from the anterior of the ovary join the vein from the fat-body to enter the postcardinal vein, while the two posterior veins from the ovary open into the main renal vein.

Figure I (b) shows the left side of the same animal. Three renal veins are present, each formed by the union of two to five smaller vessels receiving numerous irregular tributaries. The renal veins more or less parallel each other and pass forward and slightly towards the middle line. The posterior vein is by far the longest of the three, the anterior the shortest, while the middle and posterior veins are connected near their anterior extremities by a short transverse vessel. All three veins unite to form a common trunk leading to the postcaval. The anterior ovarian veins have a similar arrangement to those in Figure I (a), while the two posterior veins enter the posterior renal vein half-way along its length. The postcardinal vein is united with the anterior renal vein.

Figures 2 (a) and 2 (b) also show right and left sides of a female frog. In Figure 2 (a), the efferent renal veins are arranged in parallel series throughout the kidney, although their finer tributaries are irregular and show considerable anastomosis with each other. One main renal vein occurs, which is joined anteriorly by a smaller renal vessel. Posteriorly, a stout connecting vessel joins the main renal veins of both sides. Before the renal vein becomes continuous with the postcaval vein it is joined by the postcardinal vein. Only three ovarian veins are present, all of which make separate entrances into the main renal vein.

The condition shown in Figure 2 (b) is similar to that of the right side, except for the fact that the vein from the fat-body enters the postcardinal vein further forward and that the anterior ovarian vein enters the main renal vein after the latter has left the kidney to pass towards the middle line.

Figures 3 (a) and 3 (b) show right and left sides of a male frog. In Figure 3 (a), the main renal vein is closely attached to the kidney until it passes inward to join with the efferent renal from the opposite side, and receives approximately seven distinct, short, efferent renal vessels formed from widely ramifying tributaries. A separate but small renal vein joins the main trunk just before its entrance to the postcaval vein. The postcardinal vein enters the postcaval vein directly. The vein from the fat-body, which enters the post-cardinal, is not united with any of the four spermatic veins, all of which open separately into the short veins giving rise to the main