THE VASCULAR SYSTEM 



385 



spaces in the mesenchyma, and the mesenchymal cells surrounding them 

 are converted into the endothelial lining of the lymphatic vessel. Since 

 connexions with veins appear very early and the number of connexions 



Fig. 322. — Lymphatic vessels and veins in a rabbit of fourteen days, eighteen hours; 

 14.5 mm. The lymphatics are heavily shaded, x being a vessel along the left vagus 

 nerve and y along the aorta. The large jugular lymph sac is in contact with the internal 

 jugular vein, In.J.; it passes to the junction of the external jugular {Ex. J.) and sub- 

 clavian veins, the latter being formed by the union of the primitive ulnar, Pr.UL, and 

 external mammary veins, Ex.M. The mesenteric sac is in front of the vena cava 

 inferior {V.C.I.) and below the renal anastomosis {R.A.). Other veins include AZ., 

 azygos, v., vitelline; C, gastric; S.M., superior mesenteric; etc. The figures indicate 

 the position of the corresponding cervical nerves. X11.5. (From Bremer" s "Text 

 Book of Histology.") 



diminishes in later stages, it was formerly thought that lymphatics are 

 outgrowths of veins. Today, however, this evidence seems less con- 

 vincing than it did to earlier investigators. 



