The 'hit/in and Development of the Lymphatic System. 65 



2. Peripheral Lymphatics in Other Forms. 



The pattern of the superficial lymphatics in the human embryo is 

 shown in the two figs., 505 and 506 (Sabin 134). This was a re- 

 markable specimen of an embryo 5.5 cm. long which had been kept in 

 formalin some time. On transferring it to freshly made up alcohol, 

 air filled the entire superficial lymphatic system. It shows the primary 

 superficial lymphatic plexus. Valves in a few duets, namely, the 

 occipital, the thoracic and the inguinal, indicate the oldest vessels. 

 The differences between the superficial pattern in the human and in the 

 pig are mainly the fact that in the human embryo the thoracic lym- 

 phatics ;ire much more prominent, draining into the axilla, while in the 

 pig they drain into the superficial cervical plexus, and all the posterior 

 lymphatics drain into the inguinal region in the human embryo. In 

 the human embryo the deep lymphatics for the arm grow from an 

 extension id' the jugular sac along the primitive ulnar veins, and the 

 deep lymphatics for the legs grow from the iliac sac along the femoral 

 veins. This shows in fig. 193 (Sabin 134). 



Rabbit ami eat embryos arc like the human in having a marked 

 development of the jugular sac along the primitive ulnar vein (Lewis 

 76). In this respect they differ from the pig where the axillary lym- 

 phatics are smaller. In lie'. 7 is shown a deep vessel from the jugular 

 sac. which is, I think, the axillary duet in the pig. In the rabbit and 

 the i at the axillary sac may possibly arise independently of the jugular 

 sac, and in the rabbit a conspicuous chain of peripheral vessels grows 

 along the thoracoepigastric vein (fig. 8, Lewis 76). Injections id' cat 

 embryos show that the axillary lymphatic trunk is very large. 



The superficial pattern of the lymphatics in bovine embryos, as 

 shown by Polinski (107), is much like thai of the pig. 



In the chick the primary lymphatic vessels in the skin connect 

 the posterior lymph heart with the jugular lymph sac along the lateral 

 line in the course of the thoracoepigastric vein Mierzewski (96) 

 and E. L. Clark (27 and 29). 



VI. ORIGIN" AND DEVELOPMENT OF LYMPHATIC GLANDS 

 AND THEIR RELATION TO PRIMARY LYMPH SACS. 



It is easy to prove that the lymphatic sacs in mammals and (la 

 jugular lymph sac in birds become in part lymph glands. I shall define 

 primary lymph gland- as those which are derived from the primary 

 lymph sacs. 



