380 



The Development of the Lymphatic System 



a lymphatic duct or sac^ as there would be if the sac formed first and 

 subsequently joined the vein. 



The general course of the early development of the lymphatic system 

 in the embryo pig has been embodied in a series of diagrams, Figs. 5 to 

 11. They are made from actual injections of the lymphatics except in 

 Figs. 5 and 6, for which ihe veins were injected. All of the specimens 

 except the last two have been cut in complete serial sections. The 

 lymphatics are drawn to scale except in the first one, in which the sac 

 actually measures 228 /j. and so had to be exaggerated a little to show 

 in the diagram. 



The first diagram, Fig. 5, from an embryo 

 14.5 mm. long, shows the two lymphatic 

 buds previously described, and their relation 

 to the subclavian and cardinal veins. At 

 this stage the posterior cardinal vein enters 

 the Wolffian body. Figure 4 shows the sac 

 and the valve which guards the entrance 

 into the vein in this specimen. 



The second diagram, Fig. 6, from an em- 

 bryo 15 mm. long, shows the rapid growth 

 of the ducts anteriorly. It is now evident 

 that they open into a definite sac, corre- 

 sponding to the cervical or anterior lymph 

 heart of the frog. Small sprouts from this 

 sac have started toward the skin. In an 

 embryo 18 mm. long they have reached the 

 skin and can be injected subcutaneously. 



The third diagram, Fig. 7, is from an em- 

 bryo 2 cm. long. The lymph heart is now 

 large. Figure 3 shows its size in an embryo 

 2.5 cm. long. The ducts in the neck have now begun to spread in the 

 skin, one tuft grows up behind the ear and another grows downward 

 over the scapular area. This stage marks the beginning of the thoracic 

 duct. From the two cervical h-mphatic ducts which we have been 

 following, two ducts start out, very near the opening into the vein, and 

 grow posteriorly. The vagus nerve lies just behind the junction of the- 

 subclavian and cardinal veins and these two ducts follow the nerve on 

 either side, and lie just internal to it. These two ducts are taken from 

 sections and have not been injected at this stage. 



The same specimen shows also the two posterior lymph hearts. In 

 embryos from 15 to 19 mm. long there are no vessels in the lower part. 



Fig. 7. Diagram of the lym- 

 phatic system of au embryo pig 3 

 cm. long. X 3. Alh, anterior 

 lymph heart; plh, posterior 

 lymph heart: rid, right lym- 

 phatic duct ; td, thoracic duct. 



