462 ON THE ORIGIN AND EARLY DEVELOPMENT 



t helium with which they were connected. The vessels thus reconstructed were 

 found to be decidedly fewer in number than in the succeeding stages, and it is 

 evident that no continuous plexus was present. It was noticeable that practically 

 all of these vessels were connected with veins (fig. 32, plate 6). 



In studying the region over the pelvis in chicks of 4 days 23 hours, in sections 

 cut parallel to the surface, a very interesting condition was noted. This is the same 

 region as that shown in figure 3, in a chick of 5 days 1\ hours. In the exact position 

 of this delicate, continuous plexus of the older embryo we found a number of very 

 fine vessels which did not contain any injection mass. A great many of these vessels 

 were connected with blood-vessels. It might be inferred that such vessels are all 

 new-forming blood-capillaries were it not for the difference in their appearance and 

 location (for the most part beneath the superficial blood-vessels) and the fact that 

 similar pictures were not found in the adjacent areas in which new formation of 

 blood-vessels is also taking place. The character of these vessels is strikingly like 

 that of those present in the earliest stage of lymphatic formation in the region of 

 the posterior lymph-heart, recorded in figure 32, plate 6, and appears to show that 

 this region over the pelvis also gives rise to new lymphatic vessels, and that here 

 their origin occurs several hours later than the first formation in the tail region. 

 Evidence obtained from the operations confirm this view and will be taken up 

 later. The presence of numerous connections with blood-vessels appears to favor 

 the view that the earliest lymphatics arise as outgrowths from the blood-vessels. 

 Such communications with blood-vessels are not found in the later stages in which 

 a continuous lymphatic plexus is present in this region. 



And here we are forced to abandon the history of the first lymphatics of the 

 chick, for the stage shown in plate 6, figure 32, is the youngest in which we were 

 able to discover anything resembling a lymphatic. A chick of 4 days, measuring 

 9 mm., was sectioned parallel to the surface and this region was studied with the 

 oil-immersion for a trace of possible lymphatic endothelium, but none was found. 

 The intersegmental branches of the coccygeal veins have developed in the 4-day 

 chick, but in the region of the later lymphatic plexus we could find no endothelium 

 and no endothelial nucleus nothing but a uniformly dense mesenchyme without 

 the spaces present in later stages. 



With regard to the much discussed question as to whether these first identi- 

 fiable lymphatics arise from mesenchyme cells which differentiate into endothelial 

 cells and send back processes to the veins, or whether they are outgrowths from the 

 venous endothelium, we are unable to give a final and conclusive answer. The fact 

 that, in the earliest stages in which the characteristic lymphatic endothelium can 

 be identified, these first lymphatics are found to connect with veins in many places, 

 while in later stages the number of such connections diminishes instead of increasing, 

 appears to favor the view of outgrowth from the veins. But these studies do not 

 exclude the possibility that for a very brief period mesenchyme cells may change 

 into endothelial cells which form angioblasts and quickly acquire, first, a connec- 

 tion with the vein and then a lumen. It is to be hoped that some transparent region 

 will eventually be found that will make it possible to watch the differentiation 

 of these first lymphatics in the living and settle this question definitely. 



