The development of tlie urinogenital organs of the lamprey. 43 



appearance shown in Figs. 63 and 64. The aorta is small and 

 triangular in cross-section and I cannot find that it sends otf any 

 vessels into the trabecular tissue of the niesonephric lobes. The 

 cardinals, on the contrary, are enormous and usually distended with 

 blood. Their walls are often rather imperfect on the ventral side 

 where they give oif vessels, as on the left side in Fig. 64 and on the 

 right side in Fig. 63. Since these vessels are filled with blood- 

 corpuscles they can be traced, often for considerable distances, through 

 the trabecular tissue. They may extend to and surround the meso- 

 nephric tubules from their very inceptions. It is as if the stimulus 

 exerted by the ingrowth of the young tubules into the trabecular 

 tissue attracted the blood-corpuscles, causing them to migrate from 

 one cavity to another and finally to form definite paths, or capillaries. 

 In this stage, then, there seems to be no arterial system in the kidneys 

 but only a venous system, the blood flowing through a number of 

 anastomosing vessels which ultimately join the two posterior cardinals. 



In Ammocoetes 22 mm in length broad and frequent anastomoses 

 are formed between the two posterior cardinals beneath the aorta. 

 These anastomoses I shall call the subaortic sinus; they are shut oft' 

 now on one side, now on the other by partitions extending down from 

 the ventral wall of the aorta. 



In larvœ 7 cm long a further change which persists , at least 

 through the life of the Ammocœtes, is brought about. This is the 

 formation on either side just beneath the cardinal vein of a large 

 sinus — the subcardinal sinus (Fig. 67 sc.v). It receives the blood 

 from the capillaries of the nephric lobe and pours it into the cardinal 

 of the same side through small openings just lateral to the subaortic 

 septum. Except for these openings the horizontal lower wall of the 

 cardinal vein is well-developed. Following the sections of the veins 

 towards the tail I find that a short distance before the end of the 

 intestine the two cardinals unite to form a single vessel, the caudal 

 vein. When the latter reaches, in its backward course, the beginning 

 of the vestigial mesentery that supports the hind end of the intestine, 

 it receives a small but distinct vein from the intestinal wall and then 

 gradually tapers away into the tail. Behind this vessel from the in- 

 testine the two subcardinal sinuses also fuse to form a single vessel, 

 and this, too, communicates on its ventral side with the intestinal 

 vessels by means of a mesenteric vein. The vessel to the caudal vein 

 does not come from the spiral valve, at least not directly, because in 

 the sections containing this vessel the spiral valve is a little to one 



