302 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The anterior parts of both postcardinals have separated from the posterior por 

 tion and receive only blood coming from the intercostal veins (fig. 308). A 

 cross vessel now connects the posterior parts of the postcardinals of the two sides, 

 after which the left vessel separates into two portions. The anterior of these 

 (fig. 308, B) connects with the heart by way of the jugular and innominate vein and 

 forms the superior intercostal vein of human anatomy. The rest of the left 

 postcardinal is now known as the hemiazygos vein and it returns blood from the 

 trunk by way of a cross connexion and the anterior part of the right postcardinal 

 (now called the azygos vein), to the precava and so to the heart. 



THE LYMPHATIC SYSTEM. 



The lymphatic system consists of (i) a series of lymph vessels which 

 penetrate all parts of the body; (2) of pulsating portions of these vessels, 

 the lymph hearts; and (3) peculiar aggregates of connective tissue, 

 leucocytes and lymph vessels which are grouped under the general 

 head of lymph glands. 



There are different views as to the morphology of the blood and lymph systems. 

 According to one (Marcus) the lymph vessels were primitively connected with the 

 ccelom and have only secondarily come into relations with the blood-vascular 

 system. Others think that both blood and lymph vessels have arisen from 

 extracoelomic spaces, from which, by modification and specialization, the two 

 systems have been differentiated. The fact that in many invertebrates there is 

 but a single system, best compared with the lymph system of the vertebrates, and 

 that, even in the Crustacea, lymphatic and blood systems are but partially differ- 

 entiated, is of interest in this connexion. 



The lymph vessels are, in part, capillary in character with walls of 

 endothelium alone. The larger ducts and the still larger sinuses are 

 strengthened by smooth muscle fibres and by elastic and fibrous tissue. 

 The capillaries have numerous anastomoses, but the vessels are said 

 to terminate blindly, while, at least in the higher vertebrates, some may 

 connect with the ccelom by minute openings (stomata) in the peritoneal 

 lining. The larger vessels have valves at intervals to prevent back- 

 flow of the lymph, these often giving the vessels a lobulated appearance. 

 Proximally the vessels open at two or more points into the veins. The 

 fluid portion of the lymph is derived in part by osmose from the walls of 

 the blood capillaries, in part from the alimentary canal. 



The development of the lymph vessels has been traced mainly 

 in birds and mammals (chiefly in the latter), with fewer observations on 

 amphibia and other classes. Many points remain to be worked out, 

 there being considerable differences in the various accounts. Appar- 

 ently the process in its main features is as follows: 



