306 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
ducts, the one of the lizards being divided in front so as to empty into either sub- 
clavian vein. There is a single lymph heart at the junction of trunk and tail. In 
the birds both thoracic ducts occur and there is a pair of lymph hearts present in 
the young in the position occupied by the single heart of the reptiles. 
In the mammals the primitively paired thoracic ducts are sometime retained 
throughout life, but usually only one persists. This begins at the cistern of chyle 
in the lumbar region and empties into the left brachiocephalic vein near the entrance 
of the single cephalic duct. The thoracic duct receives the lymph vessels from the 
limbs and those (lacteals) from the alimentary canal. In those cases where there 
Fic. 312.—Early lymph system (black) of 10 mm. rabbit embryo, after F. T. Lewis. 
af, anterior tibial; c, caudal; fz, primitive fibular; ¢7, 77, external and internal jugular; em, 
external mammary; pc, postcardinal; u/, primitive ulnar veins. 
is but a single thoracic duct in front, its representative on the right side is a much 
smaller vessel connected with the right side of the venous system. No lymph 
hearts are known in the mammals. The jugular lymph sacs of the embryo have 
been regarded as such, but the absence of valves and muscles in the walls renders 
such an interpretation doubtful. 
Lymph Glands.—In connexion with the lymph vessels are numer- 
ous structures included under the heads of lymph glands, lymph 
nodules and blood-lymph glands. These are most abundant in the 
walls of the ccelom (mesenteries) and of the digestive tract, although 
they may be found at remote points. They consist of aggregates of 
