386 



The Development of the Lymphatic System 



tion of the anterior lymph heart, atid one over the crest of the ileum 

 marked c. When the ducts first reach the skin in the side of the neck, 

 one tuft grows behind the ear and the other grows over the scapular 

 area. This is shown in the area marked b, corresponding to an embryo 

 2 cm. long. At the point of radiation of these two groups of ducts a 

 plexus is formed which eventually becomes a lymph gland and cor- 

 responds to the group of glands in the posterior part of the neck of 

 the adult. 



From the anterior lymph heart another group of ducts grows to the 

 angle of the jaw and there divides, one group growing over the head 



in front of the eye and the other grow- 

 ing between the eye and the ear. This 

 second point of radiation, seen in a pig 

 3 cm. long in the area marked c in the 

 diagram, is likewise a plexus, which 

 becomes a lymph gland. A third point 

 of radiation is made in the axilla for 

 the ducts of the wall of the thorax. 

 The ducts from this plexus are present 

 in a pig -i cm. long, as is shown in the 

 area marked d. Finally, at the same 

 stage, namely, in a pig 4 cm. long, a 

 fourth point of radiation develops 

 nearer the front of the neck for the 

 superficial ducts of the arm and for 

 the ducts over the front of the neck. This plexus has been injected 

 in a pig 4 cm. long and by 4.5 cm. the ducts from it have reached 

 the middle line of the neck in front and below the elbow on the surface 

 of the arm. In Fig, 11 is seen the connection of this point of radia- 

 tion with the anterior lymph heart; it is marked arm s. 



These four points of radiation, one in the posterior part of the neck 

 or scapular, a second behind the angle of the jaw or maxillary, a third 

 in the axilla or axillary, and a fourth in the anterior part of the neck 

 or clavicular, are all connected in the depth with the anterior lymph 

 heart, and can be injected from it. The branches that radiate out 

 from the four plexuses anastomose so freely in the skin that they can 

 also be injected subcutaneously. Certainly all the ducts in the anterior 

 half of the body of a pig 5.5 cm. long can be injected by one puncture 

 over the scapular area except possibly the ducts down the arm, which 

 are too far away from the point of injection. In general, each one 

 of these four points of radiation is a plexus which develops into a 



PLH 



Fig. 12. Transverse section of an 

 embryo pig 3.5 cm. long- at the pos- 

 terior end of the Wolffian body show- 

 ing the posterior lymph hearts, to be 

 compared with Fig. 8. x 5.5. A, aorta ; 

 c, ca?lom ; Id, lymphatic ducts grow- 

 ing to the surface; plh, posterior 

 lymph heart ; v, vein formed by the 

 junction of the femoral and sciatic 

 veins as shown in Fig. 8. 



