302 COMPARATIVE MORPHOLOGY OF VFEREBRATES. 
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 (1) 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 
coelom and: have only secondarily come into relations with the blood-vascular 
system. Others think that both blood and lymph vessels have arisen from 
extraccelomic 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: 
