vi VENOUS SYSTEM 417 



blood-stream passes through these communications into the posterior 

 cardinals and the latter take on the appearance of a direct forward 

 prolongation of the caudal vein. 



Dorsally there develops on each side a cardinal trunk which 

 swells out into a great irregular sinus (jm) in the region of the 

 pronephros. On its outer side branches pass from the pronephric 

 sinus into the general vitelline network. In this network a specially 

 wide channel develops, starting from the pronephric sinus and 

 sweeping outwards over the yolk to join the lateral vitelline vein 

 and so reach the heart. This channel, which becomes gradually 

 more and more sharply defined, is the duet of Cuvier (Fig. 193, A, 

 d.C). The pronephric sinus is continued backwards into the 

 posterior cardinal vein. This is at first distinct from its fellow but 

 at an early stage fuses with it to form a median inter-renal vein 

 (Fig. 193, B, ir) — the fusion being foreshadowed by the develop- 

 ment of anastomotic connexions between the two veins while still 

 separated from one another by a distinct space (Fig. 193, A, an). 

 Towards the cloaca the posterior cardinals taper off and are con- 

 nected by irregular anastomotic channels with the subintestinal 

 vein, as already mentioned, and also with the dorsal aorta. Eventu- 

 ally, as already indicated, the inter-renal vein and the caudal vein 

 form a continuous vessel. 



During the later stages a striking asymmetry becomes apparent 

 in the anterior, unfused, portions of the posterior cardinals — the 

 left becoming greatly reduced as compared with the right. (Fig. 193, 

 D). The main blood-stream thus passes forwards on the right side, 

 and upon this side a special direct channel develops on the ventral 

 side of the pronephros, through which the blood-stream is able to 

 reach the duct of Cuvier without passing through the tangle of 

 pronephric tubules. The asymmetry affects also the ducts of Cuvier — 

 showing itself first in that of the left side becoming relatively 

 shorter than its fellow, which retains for a time its wide sweep over 

 the surface of the yolk (Fig. 193, B and C, d.C). Eventually it too 

 becomes shortened and its calibre becomes considerably greater than 

 that of the left side (Fig. 193, D). 



From the pronephric sinus a branch (l.v) develops about stage 

 30 which passes dorsal wards and then backwards beneath the lateral 

 line nerve — the lateral cutaneous vein. This, a large vessel about 

 stage 33, becomes reduced to an insignificant vestige later on. 



The anterior cardinal vein runs along the side of the head region, 

 passing through the angle on the ventromesial side of the otocyst, 

 between the latter and the brain-wall. At its front end the vein < 

 dilates into a large sinus, which gives off irregular branches to the 

 mesoderm of the head. At an early stage an anastomotic channel 

 makes its appearance on the outer side of the otocyst continuous 

 anteriorly and posteriorly with the anterior cardinal. When this 

 channel has been established (Fig. 193, A, l.c) the blood-stream from 

 the head divides in front of the otocyst and passes backwards, part 



vol. II 2 E 



