OF THE LYMPHATIC SYSTEM OF THE CHICK. 475 



This is evident from injections of normal embryos, in which the delicate plexus 

 along the side, the deep plexus near the duct of Cuvier, and the lymph-heart plexus, 

 all can be demonstrated before any lymphatics over the pelvis can be injected. 



Our studies of oil-immersion reconstructions also gave evidence of the later 

 differentiation of these lymphatics. For example, at the stage shown in plate 5 

 the lymph-heart plexus and a few extensions (probably outgrowths from it) are 

 shown as a continuous plexus. More anteriorly, in the region where the pelvic 

 plexus is found in older embryos, we were able to reconstruct a number of fine 

 vessels, some of them connected with blood-vessels but differing in appearance 

 from blood-vessel sprouts, and others apparently isolated. About eight hours later, 

 as shown in figure 3, a continuous lymphatic plexus is present throughout the region 

 over the posterior half of the pelvis, which connects with the lymphatic plexus 

 in the tail. This continues to grow rapidly in extent and richness, as evidenced by 

 the many solid processes at the edge of the plexus and the numerous mitoses in the 

 endothelial cells. The connections with blood-vessels, found in these sections in the 

 earliest stages, evidently are soon lost, since none can be injected in the later stages 

 in which a continuous lumen is present. 



These observations support the conclusion drawn from the results of the 

 operation just referred to, that there is a separate point of origin for the superficial 

 lymphatics over the posterior part of the pelvis. 



OPERATION 4. ISOLATION OF THE' ANTERIOR PART OF THE LEG AND PELVIS. 



Although operations 2 and 3 had yielded interesting results in regard to the 

 points of origin for the superficial lymphatics, a conclusive answer had not yet 

 been given to the other problem, i. e., whether lymphatics can develop in a region 

 which has been completely isolated from its normal source of lymphatic supply. 

 The evidence obtained from injections of normal embryos, and from experiment 

 3, apparently showed that the anterior portion of the leg and pelvis receives its 

 lymphatic supply from the anterior lymphatic plexus of the body-wall in the case of 

 normal chicks, or by invasion from the posterior part of the pelvis when this 

 anterior source of supply has been removed. We therefore attempted a still more 

 radical operation the effective isolation of this region. This entailed the removal 

 of several segments (three to seven) and the body- wall adjacent to them in the 

 region just anterior to the right hip. The procedure was a repetition of that used in 

 operation 3. Then, the anterior part of the posterior limb-bud, together with the 

 somites opposite, being left intact, the remainder of the leg and the adjacent somites 

 on that side were dissected away. The operation was completed, as in operation 3 

 by snipping off the tail, the aim being to completely isolate the anterior part of the 

 leg and pelvis of the right side from the area of lymphatic origin of that side, and 

 also from that over the posterior part of the pelvis. The left side of the embryo 

 was allowed to develop in a normal manner except for the absence of the tail, 

 including the lymph-heart region. 



In the first two experiments of this kind three somites opposite the anterior 

 half of the right leg were allowed to remain intact. These embryos developed 



