304 Walter A. Baetjer 



This view Avas later confirmed by Dr. Lewis* in a paper on the "De- 

 velopment of the Lymphatic System in Eabbits," published in 1905. He 

 worked out in more detail the origin of these sacs, showing them to come 

 from transformed veins. He also stated that they arose in several places 

 in the body and described two more sacs, viz., one in the root of the 

 mesentery and one along the external mammary vein. 



In the present work, which has been guided by Dr. Sabin, further 

 evidence will be offered in favor of the direct venous origin of the 

 lymphatics from a study more especially of the origin of one part of 

 this system — the mesenteric sac. For this work serial sections were 

 made of embryos from 16 mm. to 30 mm. long, in which the blood 

 vascular system had been injected, and of embryos from 33 mm. up with 

 lymphatic injections. From this time the primitive system is complete 

 and can be injected through the thoracic duct without difficulty. 



In the study of the origin of this system it has been found that there 

 are certain primitive sacs existing in different regions of the embryo 

 which represent the earliest lymphatics. As has been previously shown 

 (Sabin, Lewis, McClure and Huntingdon) the first of these appears in 

 the cervical region, near the internal jugular vein, in the pig, between 

 14 and 16 mm (Sabin). In a later stage — 21 mm. — according to the 

 reconstruction, made by Lewis, definite lymphatic vessels have arisen in 

 three regions (Fig. 8).^ 



(a) In the cervical region in association with the jugular veins. 



(&) Along the vertebral column, dorsal to the aorta, in the exact 

 location of the developing thoracic duct. 



(c) In the root of the mesentery, just ventral to the renal anasto- 

 mosis of the sub-cardinal veins. At this stage the lymphatics exist 

 independently of each other and of the veins, as definite, well formed 

 spaces in the embryonic s}Ticytium and lined by endothelium similar to 

 that of the veins.® 



Between these stages, in embryos 15 mm., in which lymphatics are 

 found only in the cervical region, and in those 23 mm., where they exist 



*Lewis. Development of Lymphatic System in Rabbits, Am. Jour. Anat., 

 Vol. V, No. 1, 1905. 



^Lewis, Ameb. Jour. Anat., Vol. V, No. 1, 1905. 



"In general, the study of my own specimens agrees with the reconstruction, 

 though I think this condition occurs at a later stage — 23 mm. — since at 21 mm. 

 the lymphatics in the region of the thoracic duct have not yet been differen- 

 tiated. However, this difference may be due to variations in the methods of 

 making measurements. 



