378 



The Development of the Lymphatic System 



subclavian vein is represented only by a constriction on the side- of the 



cardinal vein. At 14.5 mm. two visceral arches show on the surface 



and in sections the subclavian vein reaches the root of the arm bud. 



In this specimen the lymphatics are two small buds extending 228 />. 



from the vein. Figure 4 is the opening of this sac into the vein and 



shows that the entrance is guarded by a valve. 



The valves which guard the openings of the lymphatic ducts into 



the veins have required considerable study. Serial sections of five 



different stages of embryos between 

 14.5 mm. and 3 cm. have been cut. The 

 sections are stained on the slide with 

 hajmatoxylin and a combination of 

 eosin, 6 parts, aurantia, 1 part and 

 orange G, 4 parts. In this stain the 

 endothelium of the lymph ducts con- 

 trasts much better with the connective 

 tissue* than in sections stained with car- 

 mine. In all the sections of the open- 

 FiG. 4. Relation of the lymph duct ings of the lymphatics into the veins, 



to the cardinal vein in a pig- 14.5 mm. ±-1 i i. ^■ £ j- x ■ j. 



long. Aih, anterior lymph henrt. X 170. the duct lies ±or some distance against 



the vein, the two being separated only 

 by a double layer of endothelium, one for the vein and one for the 

 lymph duct. Finally, in each series one can see that, ju.st at the edge 

 of the lymph duet, these two layers are continuous, see Fig. 4. But 

 it was necessary to prove that this was actually an opening and for a 

 long time I could not inject from the lymphatics to the veins in very 

 young embryos. The reason for this is plain in Fig. 7. In injecting 

 from the periphery one is injecting through small ducts into a large 

 sac, and it is impossible to get pressure enough to force the fluid from 

 'the sac into its narrow efferent duct. One could easily fill the sac by 

 puncturing it but the vein lies too near to be sure of puncturing the 

 sac alone. However, by filling the sac carefully from the jDcripheral 

 ducts one can then press gently with the finger against the neck and 

 see that the fluid actually goes into the vein at the point of the junc- 

 tion of the subclavian and cardinal veins. It takes some pressure to 

 open the valve, and the heart should be beating in the embryo used 

 for the experiment. Having opened the valve, if serial sections are 

 cut through it, the double fold of endothelium will be found a bit 

 raised and smeared with India ink. India ink is a better fluid for these 

 injections than Prussian blue. It runs farther and easier for its gran- 

 ules are smaller and they do not clump in the lymph. This has been 

 done in embryos from 3.5 cm. down to 2.2 cm. long. 



