DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 265 



uniting with each other in a centripetal direction, until they 

 finally establish their central connection with the permanent 

 venous system. This may be briefly defined as the 'centripetal 

 theory,' In this view vascular endothelium does not possess the 

 character of 'specificity.' Vascular endothelium, wherever en- 

 countered, haemal or lymphatic, is the expression of the environ- 

 mental adaption of an originally isodiametric mesenchymal cell 

 to mechanical influences. Unilateral pressure, e.g., the accumu- 

 lation of fluid under tension in intercellular spaces, will produce 

 flattened modified mesenchymal cells, which on the one hand 

 confine the fluid, and on the other line, as a continuous layer of 

 endothelium, the tissue space in which the fluid is contained. 

 If the pressure is released, the fluid drained and the intercellular 

 space allowed to collapse, the former endothelial cell will promptly 

 revert to the type of the indifi'erent mesenchymal cell, of which 

 it is merely an adaptive form, modified in accordance with defi- 

 nite hydrostatic and other purely mechanical factors. Conse- 

 quently, in this view, the modified mesenchymal endothelial cell 

 loses all pretensions to 'specificity.' It cannot extend from the 

 region of its inception toward the periphery unless the mechani- 

 cal factors responsible for its origin precede or accompany such 

 extension. 



This second conception of the basic principles underlying \ym- 

 phatic genesis in vertebrates has in its broad general applications 

 impressed me most strongly with its truth from the beginning of 

 my iQvestigations, and I have upheld the same on several occa- 

 sions and in a number of publications. During the last three 

 years further and more detailed study of developing lymphatics 

 in mammals, and extension of the investigation on part of myself 

 and my associates to include the reptilian and avian embryos, 

 have consistently confirmed me in my earlier convictions. At 

 the same time I have realized that certain special phases of 

 lymphatic development deserve the most careful and minute 

 consideration, because the opportunities for errors in interpre- 

 tation are here most abundant, and because they present many 

 puzzling and, at first glance, contradictory conditions. 



