GENERAL CONSIDERATIONS 57 



along its entire extent; in which case a closed endothelial-lined 

 tube would result. It is also quite possible that the conditions 

 actually observed in the embryo of Chelydra might persist in the 

 adult. Whatever may ultimately be proved to be the case for 

 the peripheral lymphatics, in general, it would seem best for the 

 present to withhold an opinion until a positive demonstration is 

 at hand. 



XIII. GENERAL CONSIDERATIONS CONCERNING THE GENESIS 



AND GROWTH OF THE LYMPHATIC SYSTEM IN ITS 



RELATIONS TO THE HAEMAL SYSTEM ^ 



Does the endothelium of the lymphatic system arise, at any 

 time or place, in a discontinuous manner and independently 

 of that of the veins? As we shall see, the determination of this 

 question constitutes a solution of the lymphatic problem. 



The view that lymphatic endothelium spreads continuously 

 and uninterruptedly throughout the body of the embryo from 

 the endothelium of the veins, is merely an extension, and appli- 

 cation to the endothelium of the lymphatic system, of the well- 

 known view held by His, that the endothelium of the" intra- 

 embryonic haemal vessels grows continuously and uninterruptedly 

 into the embryo from the yolk-sac angioblast. Such a method 

 of origin necessarily implies that all intra-embryonic endothelium 

 arises only from a preexisting endothelium which takes its 

 origin in the yolk-sac, and that in the body of the embryo a 

 discontinuity of origin never occurs. 



The view opposed to the 'ingrowth' or 'angioblast' theory of 

 His has been closely associated with the names of Riickert and 

 Mollier ('06). This view consists in the claim that the endo- 

 thelium of the intra-embryonic haemal vessels develops in situ 

 in the body of the embryo, and that it is not derived from 

 the yolk-sac angioblast. 



* Section XIII has already been published in practically its present form under 

 the title of 'The development of the lymphatic system in the light of the more 

 recent investigations in the field of vasculogenesis' (McClure '15 b). 



