THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



277 



The other view * is that the lymphatic vessels are derived primarily from the 

 veins. In pig embryos of 14.5 mm. the first indication of a lymphatic system 

 is in the region of the anterior cardinal vein. At the point where the subclavian 

 joins the cardinal (jugular) a large sac-like space is seen (Fig. 252), which 

 opens directly into the cardinal by a small aperture guarded by a valve. The 

 endothelium of the lymphatic space is directly continuous with that of the vein. 

 This condition has led to the assumption that the lymphatic space is formed 

 by an evagination from the vein. A little 

 later two similar structures are found in 

 the inguinal region, one on each side, 

 which open into the posterior cardinal 

 veins (Fig. 252). These four structures, 

 known as "lymph hearts," constitute 

 centers from which the rest of the 

 lymphatic vessels develop. 



Further growth takes place by evagi- 

 nations from the "lymph hearts." In 

 the cervical region channels grow out 

 toward the skin and appear under the 

 lat er on the dorsal side of the neck, on 

 the angle of the jaw and on the ventral 

 side of the neck. From each of these 

 three areas, branches grow in all direc- 

 tions until they meet and fuse with 

 branches from the other areas on the 

 same and opposite sides. From the 

 caudal "lymph hearts," channels grow 

 out and appear under the skin on the 

 crest of the ilium and in the inguinal 

 region. From each of these areas 

 branches are sent out which anastomose 

 with branches from the other areas and 

 also with branches which have grown 

 caudally from the cervical region (Fig. 253). This brings about a vast net- 

 work of lymphatic vessels throughout the body walls. From the "lymph 

 hearts" other vessels grow out and invade the region of the aorta. A vessel 

 from the left cervical "lymph heart" bifurcates and each branch grows 

 caudally along the aorta to join corresponding vessels from the inguinal 

 "lymph hearts." In the region of the kidneys these branches become dilated 

 to form the cisterna chyli (Fig. 254). The branch on the left of the aorta 



*Sabin, Lewis. 



FIG. 253. Diagram showing network of 

 lymphatic vessels in skin of pig embryos. 

 Sabin. 



Area marked A shows extent of network in 

 an embryo of 18 mm.; B, in embryo of 

 20 mm.; C, in embryo of 30 mm.; D, in 

 embryo of 40 mm. 



