184 



The Development of the Lymphatics in the Skin 



In embryo pigs below 18 mm. in length there are no lymphatics in the 

 skin, as has been proved both by numerous negative injection experi- 

 ments and by their absence in serial sections. The first sign of the 



lymphatic system was found in a 

 pig 11.5 mm. long. It consisted 

 of two small blind ducts which 

 had budded off from the vascular 

 endothelium at the junction of 

 the cardinal and subclavian veins 

 on either side of the neck. These 

 ducts were found to grow into the 

 neck along the anterior cardinal 

 or jugular veins to a point mid- 

 way between the ear and the scap- 

 ula, and here widened into a sac. 

 This sac, though possessing a 

 lining of a single layer of endo- 

 thelial cells without a muscle coat, 

 I have considered to be analogous 

 with the lymph hearts of the am- 

 jDhibia. From the sac the ducts 

 grow directly outward to the skin, 

 which they reach when the pig is 

 18 mm. long. 



In Figs. 1 to 5 is given a series of actual injections of the lymphatic 

 ducts in the skin of pigs of increasing sizes. Each picture is a drawing 

 from one actual injection, and all of the injections are practically com- 

 plete except Fig. 3. That is to say, there are no lymphatics in the skin 

 at these various stages excepting those which are shown injected. The 

 methods of these injections are given in the paper cited above. 



Fig. 1 represents the lymphatic ducts in the side of the neck of a 

 pig 2.5 cm. long, and shows that the ducts are growing in two directions, 

 first over the back of the head behind the ear, and secondly over the 

 scapular region. In Fig. 2, from a pig 3 cm. long, these two tufts of 

 lymphatics, one behind the ear and the other over the scapula, are more 

 distinct and have increased in complexity. A new set of ducts has 

 reached the surface at the angle of the jaw and has begun to grow out 

 in two directions, first between the eye and the ear, and secondly in front 

 of the eye. 



Fig. 3, from a pig 3 cm. long, does not show the entire lymphatic sys- 



FiG. 1. The lymphatic system in the 

 skin of a pig 2.5 cm. long, x 3. 



