260 GEO. S. HUNTINGTON 



lymphatic system is fairly well known in the Mammalia. Very 

 important contributions to this subject have recently been made 

 by McClure and Silvester (1, 2). Our knowledge of the adult 

 anatomy of the lymphatic system in the remaining amniote 

 classes is less complete. On the other hand, the development 

 of the lymphatic vessels, in spite of the great progress made in 

 the last ten years, still presents unsettled questions, chiefly 

 because opinions differ as to the genetic relations between lym- 

 phatic and blood vascular channels. 



The primary questions, which form the basis for the existing 

 differences of opinion, involve the origin of the lymphatic appa- 

 ratus in the broad sense. Is the same an independent vascular 

 development, coequal to and largely concurrent with the develop- 

 ment of the blood-vascular system, or is it a secomdavy derivative 

 from the venous division of the ha)emal system? These are the 

 two fundamental questions. There are, of course, many sec- 

 olidary problems which arise at nearly every point in the course 

 of the inquiry into the lymphatic ontogeny of the vertebrates, 

 such as differences in the types of development encountered in 

 embryos of different vertebrate classes, differences in the grade 

 of development, number, structure and location of the lym- 

 phatico-venous hearts and of the equivalent lymphatico-venous 

 connections, and many other problems of minor genetic signifi- 

 cance. But in the last analysis the question of the independent 

 origin of the lymphatic system, or of its secondary derivation 

 from the veins, remains the cardinal and crucial point upon which 

 the true genetic interpretation of lymphatic organization must 

 be based. We have now, I believe, arrived, on the hand of new 

 and cumulative evidence, at a point where a careful revision of 

 some of our earlier results will materially aid in clarifying the 

 main problem. A brief review of the principal divergent views 

 held regarding the development of the vertebrate Ijrmphatic sys- 

 tem may serve to accentuate the significance of the facts I desire 

 to present at this time. 



I. The results obtained by the injection of embryos have led 

 some observers to advance — and still maintain^^a theory of 

 lymphatic development based on the following postulates: 



