Florence E. Sabin 



381 



of the body which are not definitely blood-vessels proved by injections. 

 In the embryo 2 cm. long there are two lymphatic sacs connected with 

 the posterior cardinal vein below the Wolffian body. At this point 

 the sciatic and femoral veins unite in the posterior cardinal vein which 

 enters the Wolffian body as the renal portal system. The two sacs, 

 corresponding to the posterior lymph hearts of the frog, lie close to 

 the vein, and have already sent sprouts 

 out toward the skin. Their opening into 

 the vein is also guarded by a valve so that 

 they remain empty in a complete venous 

 injection. Figure 12 is a section passing 

 through the posterior lymph hearts in a 

 pig 2.5 cm. long, and shows also the ducts 

 reaching part way to the skin. 



The next stage. Fig. 8, is from a pig 

 2.? cm. long. It shows a fnrther develop- 

 ment of the superficial lymphatics. At 

 the same time it is the fundamental stage 

 in the development of the thoracic duct 

 and explains the asymmetry of the lym- 

 phatic system. The duct which grows 

 downward on the left side meets the aorta 

 and relying on its support grows more 

 rapidly than the duct on the right side l/...\ .....V. ...^^ 

 and so becomes the thoracic duct. It 

 passes downward not only on the left side 

 of the aorta but also sends a branch 

 behind the aorta to pass downward on 

 the right side. This may be called the 

 right thoracic duct, to distinguish it from 

 the right lymphatic duct. The duct which 

 started out from the cervical duct on the right side remains as the right 

 lymphatic duct, and in serial sections of a pig 3 cm. long has been traced 

 to the root of the right lung. At this stage there are two buds start- 

 ing from the left thoracic duct toward the heart and the lung. The 

 thoracic ducts shown in this diagram as well as in all the succeeding 

 ones, are from injections. 



The method of injecting the thoracic duct is as follows. Occasion- 

 ally, it is possible to inject the thoracic duct completely from the peri- 

 phery in a pig about 4 cm. long. Fig. 10. In general, however, in 



Fig. 8. Diagram of the lym- 

 phatic system in an embryo pig 2.T 

 em. long. X 3. Alh, anterior 

 lymph heart; d, diaphragm; h, 

 ducts to the heart ; lu, ducts to the 

 lung. 



