The Lymphatic System in Human Embryos. 55 



first show no vessels in the mesentery, next vessels inject from the 

 sac into the mesentery and these vessels gradually extend to the bowel 

 wall which they reach in embryos between 4 and 4.5 cm. long. Thus 

 injections of the retroperitoneal sac make it possible to trace the 

 development of the lymphatics to the viscera, and this is an im- 

 portant point in the proof of the general theory. 



In 1904 F. T. Lewis published an important contribution to our 

 knowledge of the lymphatic system. He studied perfect serial 

 sections of rabbit embryos, worked out the early history of the jugu- 

 lar sacs, and discovered the retroperitoneal sac as has been mentioned. 

 In studying the peripheral lymphatics, Lewis pictured a series of 

 small isolated vessels extending along the external mammary and 

 umbilical veins. These isolated vessels are distinguished in sections 

 by being slightly larger in caliber than the neighboring vein. In 

 sections they are clearly isolated. I have had the privilege of examin- 

 ing Dr. Lewis' specimens and can confirm his observations entirely. 

 In one or two places there was evidently great difiiculty in determin- 

 ing w^hether some of these vessels were isolated or were connected 

 with the vein. Moreover, I can find some of these isolated vessels 

 in pig and human embryos. These numerous lymphatic anlagen of 

 Lewis are now the crucial point in connection with the lymphatic 

 system. They exist undoubtedly in perfect sections, they are always 

 lined by a perfectly formed endothelium and never show any tran- 

 sitions toward tissue spaces. The question is simply, Are they lym- 

 phatic vessels which have grown from the sacs and are only apparently 

 isolated or do they arise in situ? I believe them to be true lym- 

 phatics derived from the sacs and will give my reasons shortly. 



In connection with Lewis' observations it is important to make 

 clear the work of Huntington and McClure.^" They have strength- 

 ened the theory that the lymphatics come from the veins the more 

 because they began with a vigorous attack upon the theory. 



In 1906 they described elaborate models of the developing lym- 

 phatics in cat embryos which showed lymphatic vessels along the veins 

 previous to the formation of the lymph sacs. These early lymphatics 



-"Huntington and McClure. Anat. Record, Vol. I. 1906-Or, and Vol. II, 1908. 



