64 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



would he now be justified in claiming that all of the remaining 

 blood vessels of the embryo were formed in the same manner? In 

 view of the fact that we now know that intra-embryonic blood 

 vessels are not all formed in this manner, it would seem that a 

 a similar interpretation might also apply to the lymphatics 

 (see E. R. Clark, 'lib). 



Whatever else the case may be, in view of the above-mentioned 

 experimental investigations of Hahn, Miller and McWhorter, 

 Reagan, and Stockard, it can now be definitely stated that the 

 endothelium of the lymphatic system is neither directly nor 

 indirectly a product of the yolk-sac angioblast. Such being the 

 case, it must either arise in situ, like the endothelium of the 

 intra-embryonic veins, from cells other than from a preexisting 

 endothelium; or, be a product entirely of the endothelium of the 

 veins. If the former case be true, the endothelium of the lym- 

 phatic system should present exactly the same independent and 

 discontinuous method of origin in the embryo as that of the extra- 

 and intra-embryonic haemal vessels; and, if the development 

 of the lymphatics were followed by the injection method, the 

 same restrictions as regards the injectibility of its independent 

 anlagen should also necessarily apply. On the other hand, if 

 the lymphatic system is entirely a product of the endothelium 

 of the veins, its origin from mesenchyme should naturally never 

 occur. As a matter of fact, since intra-embryonic vascular endothe- 

 lium has been shown by experiment to be a local product of mesen- 

 chyme, there now remains no valid reason or significance in the 

 claim, as regards its specificity, that lymphatic endothelium is solely 

 a product of that of the veins. 



Let us examine the evidence at hand and see whether the endo- 

 thelium of the lymphatics, like that of the haemal vessels, develops 

 in situ in the mesenchyme, or whether it forms an exception 

 to that of the haemal vessels, and sprouts throughout the body 

 of the embryo from an endothelium (venous) already formed. 



It is not the purpose of the present discussion to give an his- 

 torical review of the literature bearing upon the development of 

 the lymphatic system but merely, on the basis of comparison, 

 to call attention to the evidence in favor of the view that the 



