DEVELOPMENT OF PETROMYZON FLUVIATILIS. 343 



currents can be seen, and along the top of each myomere a 

 backwardly directed stream. In the tail the aorta splits, and 

 one branch passes each side of the cloaca ; they unite ventrally, 

 and are continued forwards as the subiutestinal vein. Before 

 it splits it gives off a vessel which runs back along the base of 

 the notochord to supply the tail; this may be termed the 

 caudal artery. The blood from this is returned by a caudal 

 vein which soon splits into the two posterior cardinal veins. 

 These large veins run forward, one each side of the aorta : 

 the duct of the pronephros runs in their wall. Anteriorly 

 they unite with the anterior cardinals, and form two ducts of 

 Cuvier which open into the sinus venosus. The anterior 

 cardinals bring back blood from the head. The tubuli of the 

 pronephros lie in their cavity, so that the pronephros, like the 

 kidney of the Amphibia, has a double blood supply. The car- 

 dinal veins do not appear till after the subiutestinal vein, which 

 for some little time is the only vein in the body. Later still a 

 vessel appears in the right side of the intestine, opposite the 

 subiutestinal vein in the spinal fold; this, like the last named, 

 passes through the liver. In my latest stages also there is 

 an impaired vessel bringing blood back to the heart from the 

 ventral region of the gills ; this is mentioned by Balfour. 

 The blood-corpuscles are of only one kind, large oval disc-like 

 structures, with a well-marked nucleus. The protoplasm 

 scarcely stains, but the nucleus assumes a deep colour. 



Owing to the transparency of the larva, the circulation can 

 be watched with great ease. The walls of the vessels at first 

 possess no elasticity, hence great regurgitation takes place, 

 and the blood advances by a series of jerks. The valves at the 

 anterior end of the ventricle and between the auricle and the 

 ventricle prevent this affecting the blood in the heart. 



The heart begins to beat long before the cells exhibit any 

 histological differentiation into muscles. 



The Pronephros. 

 The first origin of the larval excretory system is by no means 

 easy to make out, a* it arises at a period when the embryo is 



