OF THE LYMPHATIC SYSTEM OF THE CHICK. 479 



3. Such points of origin are not restricted in size; in fact the areas in which 

 differentiation of superficial lymphatics takes place appear to be fully as large as 

 the regions which are supplied by ingrowth. 



4. When a region of origin is removed by operation, at a stage before lymphatics 

 have started to develop, the adjacent region usually supplied by extension of lym- 

 phatics from this point of origin receives its lymphatic supply by ingrowth from 

 another source (operation 1). 



5. When the point of origin removed is one of those in which venous connec- 

 tions are normally retained (such as the posterior lymph-heart region) no new per- 

 manent connections with veins develop. In this case a beating lymph-heart never 

 develops elsewhere to replace the one whose anlage has been removed (operation 1). 



6. When a region is effectively isolated from all its usual sources of origin, 

 lymphatics will eventually develop in situ (operation 4). 



In a word, the results of the operations justify the general conclusion that, 

 while the formation of permanent connections between the lymphatic system and 

 the veins is apparently restricted to certain definite regions, the differentiation of 

 lymphatic endothelium is not so restricted, but occurs at many places in the embryo . 



GENERAL CONCLUSIONS. 



The modern work initiated by the studies of Ranvier, Sabin, and MacCallum, 

 has shown that the lymphatic system is composed of definite vessels which are 

 everywhere separated from the spaces and cells of the connective tissue by an endo- 

 thelial membrane. The discovery that this membrane is present throughout the 

 lymphatic system has made necessary a revision of the older terms for certain 

 fluids of the body, all of which had been known as lymph, and a new division into 

 (1) plasma, (2) tissue fluid, and (3) lymph, designating respectively the fluid inside 

 the blood vessels, the fluid of the tissue spaces (including the cerebro-spinal fluid 

 and the fluid of the serous cavities) and the fluid inside the lymphatics (Sabin, 

 1916) . The basis for this new conception of the lymphatic system, with the emphasis 

 which it throws upon the importance of endothelium, rests chiefly on the embryo- 

 logical studies begun by Sabin and carried on by numerous other investigators. 

 These morphological studies have shown that lymph-vessels invade the different 

 regions and organs of the embryo by a process of ingrowth, and observations on 

 the transparent tails of living tadpoles have established the fact that lymphatic 

 capillaries remain completely independent of the surrounding cells and tissues and 

 grow by a process of sprouting from preexisting lymphatic endothelium. It has 

 also been shown, in studies of chick embryos, that the primitive form of the lym- 

 phatic system is an indifferent, net-like plexus, out of which ducts and sacs develop 

 secondarily in response to pressure conditions inside and outside the vessels. A 

 beginning has been made toward finding out some of the reactive powers of lym- 

 phatic endothelium : it is known to be phagocytic and to be capable of responding to 

 certain substances (by growing toward them) which do not stimulate a similar 

 response on the part of the blood-capillaries. 



