8o GENERAL SKETCH OF 



in the connective tissue external to each head-kidney. In the 

 larval ling the large channel from the liver to the tail, con- 

 taining numerous large round corpuscles, can be traced on the 

 fourth day. The venous trunks in front of the pectoral fins 

 send prolongations downward and communicate with the 

 venous end of the heart, which at this time is directed upward 

 and backward as a sinus venosus which receives the venous 

 trunks (Ductus Cuvieri) on each side, the latter trunk receiving 

 not only the cardinal vein but the jugular. Around the cardinal 

 veins the cell-tissue of the head-kidney or pronephros gi-ows. 



Before the end of the first week after hatching a simple 

 circulation can be detected. The anterior bulbus and ventricle 

 of the heart drives the blood upwards behind the eyes, pro- 

 bably by the artery of the hyoid arch, and along the dorsal 

 aorta to the root of the tail, there it forms a loop and returns 

 by a large venous trunk which anteriorly splits into the two 

 cardinals. A day or two later a venous branch, the sub- 

 intestinal, leaves the sub-vertebral vein and passes to the 

 lower side of the gut along the margin of the liver to the sinus 

 venosus. The intestinal artery (coeliaco-mesenteric) has a much 

 longer course, for it leaves the dorsal aorta in the posterior region, 

 traverses the mid-gut in descending, then courses beneath the 

 rectum, ascends before reaching the vent, and passes along 

 the anterior margin of the urinary vesicle to join the cardinal 

 vein. In a cod of the seventh day the aorta and vein reached 

 nearly a quarter the length of the cardinal trunk, while on the 

 fourteenth day they extended almost to the tip of the tail. 

 As was observed in the salmon many years ago by one of 

 us the force of the current seems to hollow out the yielding 

 channel. During the second week there are two gill-arches 

 and sometimes three, which have developed arterial channels, 

 and another vessel, the hyo-opercular artery. The mandibular 

 courses along the ventral margin of the mandible, the vessels of 

 opposite sides meeting and returning by a single vein. In the 

 cod, when the caudal artery extends f the length of the tail, 

 four branchial arteries are visible, and a sub-maxillary artery 

 beneath the eyes, whilst a return current is directed over the 

 eyes by the supra-ocular vein. The vessels of the abdomen — 



