6 VASCULOGENESIS IN THE CAT 



deny that endothelial cells undergo mitosis, that vessels after 

 formation lengthen, or that they maj' branch and send out sprouts; 

 for it by no, means implies that endothelium does not grow, but 

 it relegates these phenomena to a later period in the life of the 

 embryo, and endeavors to distinguish between a cytomorphosis 

 terminating in a definable type of cell (development, genesis, 

 anlage, das Werden) and a proliferation and extension of a fully 

 formed tissue (growth, das Wachsen). 



One further point, the great vascularity of the embryo as com- 

 pared with the adult makes the fate of the temporary vessels 

 and their endothelium of some importance to a general theory 

 of this tissue. The adaptive view considers that they revert to 

 mesenchyme, the doctrine of specificity that they disappear. 



Divergent principles of interpretation rather than differences 

 in method and technique underlie, I believe, these discrepant 

 theories, although I am aware that the contrary has been em- 

 phatically proclaimed, and in particular that the method of in- 

 jection has been held to be the ultunate criterion in these matters. 

 Without discussion at this point of what I believe to be the claims 

 of enthusiasm, it is still possible to admit the objective reality of 

 the positive data obtained by this method, without attaching 

 finality to its negations. One may admit that where the injec- 

 tion passes, there is a channel, without denying that beyond the 

 limit of injection there are forming channels, or considering it 

 proved that injected channels are always lined by a complete 

 endothelium. One may admit that the blind terminals of \'es- 

 sels are actual terminals, without assimiing that they are always 

 sprouts, for they may be regressiA'e. And above all, one may 

 admit the frequency of extravasation, without holding it to be 

 ohne weiteres an artifact. But if these reser^rations are allowed, 

 or even conceded to be still matters of free and unbiased inquiry, 

 the positive results of the injection method will form but a slender 

 basis for such imposing doctrines as those of the specificity of 

 endothelium, and of the angioblast. 



From the time of the establishment of the ch-culation, the body 

 of the embryo and some of its membranes are supplied with con- 

 tinuous channels which distribute the circulating medium, and 



