BEHAVIOR OF ANURA TOWARD COLLOIDAL DYES 53 



endothelium of the veins. The circumstance that lymphatics have 

 actually been observed to sprout and grow from other lymphatics 

 constitutes, therefore, the chief evidence upon which the theory 

 of the venous origin of the lymphatics rests. 



The adherents of this view might similarly infer from the growth 

 of blood-vessels in the fin of the tadpole's tail, or elsewhere, that 

 all other blood-vessels in the body must also originate by growth 

 from some pre-existing endothelium, whose source of origin must 

 of necessity be the unit vascular anlage designated by His as the 

 'angioblast.' As a matter of fact, this is exactly the line of rea- 

 soning followed by them. The assumption of a unit vascular 

 anlage as a source of all vascular endothelium, both haemal and 

 lymphatic, is the only logical conclusion of such a theory of general 

 endothelial growth. 



Recent experiments 6 have shown, however, that a unit vas- 

 cular anlage, the angioblast, as a source of all endothelium, is a 

 myth and does not exist. What has been found to exist is that 

 endothelium arises discontinuously in situ from mesenchyme, 

 throughout the embryo, and in the exact location in which recog- 

 nized vessels appear. The assumption, therefore, upon which the 

 theory of a continuous growth of endothelium throughout the 

 embryo is founded, has been proved by experiment to be incorrect. 



How can we reconcile the fact that intraembryonic endothe- 

 lium does arise in situ from mesenchyme with the fact that it has 

 also been observed to sprout and grow? The answer to this 

 question by adherents of the continuous growth theory should of 

 necessity be a flat denial that such a condition could conceivably 

 exist. We now know, however, that such a condition actually 

 does exist, and there must be some plausible explanation for it. 



As a matter of fact it would be inconceivable to imagine endo- 

 thelium incapable of growth. How could we otherwise explain the 

 growth of the vascular system from the embryonic to the adult 

 stage? We can reconcile the fact of a local differentiation of 

 mesenchyme into endothelium with the latter's ability to grow, 

 on the recognized grounds that endothelium, after its differ- 



6 The reader is referred to the works of McClure, '15, '16 and Reagan, '17 for 

 the literature relating to these experiments. 



