CH. vi VEINS OF POLYPTERUS 419 



on its outer and part on its inner side. Eventually tin- inm-i 

 channel becomes constricted across and finally completely severed at 

 its liindiT end so that the whole blood-stream passes back by what 

 was the outer channel. The inner channel persists as a small vein 

 which opens at its front end into the definitive anterior cardinal. 

 We see then that here, as in the Lung-fish or the Elasmobranch, the 

 " anterior cardinal " vein of later stages has intercalated in its length 

 a segment of lateral cephalic vein. 



An inferior jugular vein on each side (i.j) drains the blood from 

 the ventral side of the head into the duct of Cuvier, and as develop- 

 ment goes on these become asymmetrical, the left becoming greatly 

 reduced while the right forms a large vessel, trifid at its front end 

 where it receives the blood from the thyroid (Fig. 193, D, Th). 



The liver rudiment, at first a portion of the general yolk-mass, is 

 at this stage supplied with blood by the portion of the general 

 vitelline network which extends over it. The portal vein develops 

 simply as an enlarged channel of the network on the left side of the 

 hepatic rudiment. There is no information as to whether this is 

 really the persisting left lateral vitelline vein as we might expect : 

 nor is it known whether the hepatic vein is derived from the pre- 

 hepatic portion of this same vein. 



An important feature is that there becomes established an 

 anastomosis between the blood-vessels of the liver at the hinder end 

 of that organ and the inter-renal vein, so that a portion of the inter- 

 renal blood - stream becomes short - circuited direct to the heart 

 through the hepatic vein, which in correlation with this forms a wide 

 channel throughout the length of the liver (Fig. 193, E, h.ty.v.c')). 

 This enlarged hepatic vein is of morphological importance as it is 

 clearly the equivalent of a posterior (or " inferior ") vena cava. In 

 the Crossopterygian this vessel takes a step in evolution beyond the 

 condition in Lung-fishes, inasmuch as its posterior portion becomes 

 denuded of liver substance, so that it runs for a considerable distance 

 through the splanclmocoele as a naked vessel. 



Another important vein which makes its first appearance in the 

 Crossopterygian is the pulmonary vein. The hepatic vein, which 

 towards its headward end lies on the dorsal side of the liver, comes 

 into close contact with the pharyngeal floor in the region of the 

 glottis, and venous spaces developing in the mesoderm sheath of the 

 pharynx come to open into it. These are apparently the forerunners 

 of the pulmonary vein. In the 30-mm. larva, as in the adult, there 

 is a main pulmonary vein which still opens into the hepatic vein on 

 its dorsal side (Fig. 193, D, p). In addition to this main pulmonary 

 .vein, formed by the fusion of two branches coming from the ventral 

 side of the two lungs, there is a small accessory vein coming from 

 the dorsal side of the root of each lung and from the adjoining parts 

 of the pharyngeal wall. These also open into the hepatic vein just 

 in front, and on each side, of the opening of the main vein. 



TELEOSTEI. Amongst the Teleostean fishes we find the same 



