426 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



systemic network of capillaries interposed between them and the 

 heart. Further the quality of the blood supplied in this way to the 

 kidney is betters-being arterial instead of venous — and for both these 

 reasons we can readily understand the tendency in the more highly 

 developed vertebrates for the renal portal system to disappear. 



It has already been remarked that the posterior cardinals do not 

 pass back into an unpaired caudal vein as in the Lizard. A vestige 

 however of the unpaired condition may perhaps be recognized in the 

 development of an anastomosis between the two vessels just behind - 

 the metanephros. From the transverse bridge so formed a connexion 

 (coccygeo-mesenteric vein) is established with the portal vein in the 

 mesentery. 



The anterior cardinal vein together with an intercalated portion 

 of lateral cephalic persists as the jugular vein of the adult bird. 



The posterior cardinal vein undergoes in the Bird a curious 

 change of position in relation to the root of the iliac artery which it 

 crosses behind the mesonephros. At first it lies on the ventral side 

 of this artery : then it develops an accessory channel round the 

 dorsal side of the vessel and finally the whole blood-stream passes by 

 this dorsal channel while the ventral one disappears. This affords a 

 good example of the way in which a vein may in the course of evolu- 

 tion pass an apparent barrier formed by an artery, nerve or other 

 organ. 



Inter - segmental Veins. — In the body wall there develops a 

 series of veins corresponding with the intersegmental arteries and 

 opening into the cardinal veins. 



Veins of Limbs. — The vascular network of the limb-bud drains 

 into the posterior cardinal vein. In the Bird (Evans, 1909) the 

 drainage during its earliest stages is into the allantoic vein. Later 

 numerous channels arise connecting the network with the posterior 

 cardinal and presumably one or two of these become enlarged and 

 persist as the definitive veins draining the hind limb. 



Lymphatic System. — The venous system has its obvious roots 

 peripherally in the capillary network of blood-vessels, but it is also 

 provided with a much less conspicuous set of tributary channels 

 which constitute the lymphatic system. This extension of the 

 vascular system retains a lower grade of evolution than the remainder. 

 Its channels are less sharply defined, the lining endothelium over 

 most of its extent having a much feebler development of the backing 

 of connective tissue and muscle which forms the thick wall of the 

 vein or artery. In its peripheral portions the lymphatic spaces may 

 have remained practically in the primitive condition of intercellular 

 chinks of the mesenchyme, while in its central portions, as it 

 approaches the points at which it opens into the ordinary veins, its 

 walls may be well developed and muscular. The lymphatic system 

 serves to drain off the plasma which has oozed out from the capillary 

 blood-vessels and forms the internal medium bathing the surface 

 of the living cells of the body, and to return it to the blood-stream. 



