LYMPH-FLOW AND LYMPHATICS, CHICK EMBRYOS 417 



its size it appears probable that a circulation of lymph from the 

 deep axilla and the under part of the wing, into the deep anterior 

 plexus is still present. The tissue over these vessels is too thick 

 to test this point by watching the movement of injected granules. 

 Posterior circulation of granules into the deep jugular plexus 

 can be demonstrated by injection into the deep cervical lym- 

 phatic which accompanies the jugular vein. 



In chicks of 7^ days a new plexus of capillaries grows posteriorly 

 from the supra-scapular lymphatics. These vessels spread out 

 over the dorsal part of the side region and run parallel to the 

 main side channel. Toward the end of the 8th day of incubation, 

 granules injected into these dorsal side lymphatics circulate 

 anteriorly into the supra-scapular lymphatics, and from them 

 they enter the deep jugular plexus through the connection ante- 

 rior to the shoulder. Thus the circulation, in these lymphatics 

 dorsal to the shoulder, is again reversed. For several hours 

 the old main side channel persists and granules injected into 

 the ventral side lymphatics circulate posteriorly over the pelvis 

 and into the lymph heart. This is a transition stage between 

 those illustrated in figures 5 and 6. 



In embryos of 8 days and over, the new dorsal path of drainage 

 for the side becomes more and more prominent and gradually 

 supplants the original side channel as the main path of cir- 

 culation. Granules injected anterior to the hip, all move ante- 

 riorly following a fairly direct path into the supra-scapular lym- 

 phatics. Here their progress becomes slow and their passage 

 into the deep lymphatics is greatly delayed. For the region over 

 the pelvis, the lymph flow is still posterior into the lymph heart. 

 Injections of this stage show that lymph trunks are present over 

 the pelvis and also over the dorsal part of the side and that these 

 vessels correspond in position and size to the course and rate of 

 the lymph-flow as tested by the granules injected in the living. 

 The injections also show in the supra-scapular region where the 

 movement of granules in the living embryo, was slow, distended 

 sac-like vessels (fig. 6). That the pressure in these vessels is 

 very high is evident from the ease with which the injection mass 

 extravasates. Occasionally, in living chicks, after the injection 



