48 DEVELOPMENT OF THE LYMPHATICS OF THE LIXCIS L\ THE EMBRYO PIG. 



theory is correct, it is clear that it should be iK)ssible to follow the growth of lym- 

 phatics from the sacs into any organ or group of organs. It shoukl also he i)ossible 

 to demonstrate in i)r()gressively older stages constantly increasing lymphatic zones 

 and decreasing non-lymphatic zones. The development of the lymi)hatics of the 

 skin, of the intestine, and of the lung has now been studiinl in this manner. 



In 1904, Dr. F. R. Sabin demonstrated that the skin receiv(>d its lym])hatic 

 supi)Iy from the two jugular sacs and the two iliac sacs. From each of these sacs 

 a group of radiating vessels invade the skin and form there a close-meshed plexus. 

 These four plexuses gradually increase in size and (inall\- unite, so that the entire 

 skin is supplied with lymi^hatics. The differentiation wliich takes place varies with 

 the location and depends upon the adaptation which the vessels must make to the 

 other structures. Continuing the work of Baetjer (1908) on the retroperitoneal 

 sac, Heuer (1909) studied the development of the intestinal lymphatics by the 

 injection of this sac. He observed and described progressive changes in the intes- 

 tinal supply, finding more complex injections possible in each older stage. He inter- 

 jjreted these results to mean that the lymphatics had not extended beyond the point 

 which his injections reached and that the region beyond this point constituted a 

 non-lymphatic zone. 



There is, therefore, a primary and a secondary lymphatic system. The former 

 consists of a series of sacs formed from the veins and connected by the right and 

 left thoracic ducts. The secondary system consists of the peripheral vessels, which 

 are held by some to be outgrowths from the sacs and by others to be formed in situ. 

 With regard to the development of these i^erii^heral vessels, only those of the skin 

 and the intestin(> had been studied. There was need, therefore, for the study of 

 the other abdominal and the thoracic lymphatics. This work was begun to establish 

 a clearer conception of the develojjment of the secondary system. 



In presenting this study, I do not claim to have found any new evidence as 

 to the mode of growth of lymi)hatics. This work supports the centrifugal theory 

 in the same manner as does that of Heuer (^1909) ; and it is certain that the theory is 

 sufficiently well established to serve as a basis for this work. It is the object of 

 the present paper to follow the gross morphological changes in the development of 

 the lymphatic vessels of the lung from the ])rimary stage to the adult form. It is 

 desired to indicate the general lines of growth and the various stages which the 

 system passes through in the course of its development. No attempt has been 

 made to study the finer structure of the vessels or the mode of growth. 



It is important to note that complete injections are very difficult to make, and 

 that it is also difficult to be certain whether the injection in a particular specimen 

 is complete or not. Therefore it is not claimed that any of the injections are com- 

 plete; and the limits of the lymphatic and the non-lymphatic zones at any stage 

 are defined in a general manner, depending on the comparison of a number of 

 .specimens. 



The lymphatic supply of the lungs develo])s from three sources: the thoracic 

 duct, the right thoracic duct, and the cejjhalad portion of the retroperitoneal sac. 

 In 1913, i^abin remarked: "The right lymi)hatic duct curves ventralward and grows 

 to the heart and lungs." This is the onh' statement which I have been able to find 



