LYMPH-FLOW AND LYMPHATICS, CHICK EMBRYOS 419 



of curving around ventrally and entering the deep plexus anterior 

 to the shoulder, at a gradual incHnation, it now comes off at 

 right angles to the superficial lymphatics and pursues a steep 

 circuitous course. It seems evident from an examination of 

 chicks of this stage that the shifting of the deep lymphatics and 

 the growth of the shoulder cartilages have interfered with the 

 free outlet of the lymph from the superficial lymphatics and in 

 consequence the fluid has become dammed back. At this stage, 

 the subcutaneous tissue is very loose and apparently the endo- 

 thelium encounters very little resistance from outside and ex- 

 pands in response to the increased pressure from within, resulting 

 in the formation of a large lake or sac. 



At this stage also the posterior lymph heart first assumes the 

 form of a sac. The muscular walls appear to offer an obstacle 

 to its expansion for its size is very little larger than in chicks of 

 7 days. The separate channels which formed the cavity of the 

 heart in earlier stages have enlarged still further until the walls 

 between them are represented merely by thin trabeculae. The 

 venous connections of the lymph heart have now been reduced 

 from five to three — those communications nearest the channels 

 which differentiated from the primitive plexus are the ones which 

 are retained. 



Cleared specimens of injected embryos show that the lymph 

 heart, at this stage, receives tributaries from an extensive plexus 

 of lymphatics around the aorta and also from an extremely rich 

 plexus of allantoic lymphatics. The early development of these 

 allantoic vessels has been studied but it cannot be considered 

 at this time. At 8 days, these lymphatics have extended for a 

 considerable distance along the main allantoic blood vessels, 

 and ink injected into them in living chicks can be seen to move 

 rapidly toward the embryo, and a few seconds later the lymph 

 heart becomes dark with the ink which has reached it in this way. 

 The size of the channels which connect these allantoic lymphatics 

 with the lymph heart show that there is a very active passage of 

 fluid through the lymph heart, from sources other than the 

 superficial lymphatics, and probably accounts for the expansion 

 and coalescence of the channels composing the lymph heart. 



