36 VASCULOGENESIS IN THE CAT 



More recently Bremer"' has reconstructed what is essentially 

 a prevascular stage of this layer. 



I would emphasize the early stage and the cells which do not 

 become endothelium, because I believe that in them we find an 

 exit from the embarrassment of recognizing an independent angio- 

 blast which yet in its products is identical with mesenchyme, 

 for we have too little evidence of convergence in ontogeny to make 

 the idea of such an assimilation a priori attractive. It would seem, 

 therefore, sunpler to acce])t an origin of mesenchyme in the 

 splanchno-pleure at an earlier period than has been universally 

 recognized, by a migration of elements from the germ layers into 

 the mesostroma as has been described by v. Szily," Studnicka" 

 and others. The peculiarity of this district would then consist 

 in its great degree of vascular productivity which might be occa- 

 sioned physiologically by its relation to the yolk," rather than 

 determined genetically by a hypothetical derivation from the* 

 entoderm. We here approach the standpoint of Raffaele**" — 

 "le cellule endotheliali provengono dal mesenchifne" — and are encour- 

 aged to hope that eventually the formally genetic views embodi- 

 ed in the doctrine of the angioblast will give way to a more I)io- 

 logical standpoint of interpretation, viz., that endothelium is the 

 result of mechanical, blood of biochemical factors, both conse- 

 quent on position in the broad sense of the term, and are neither 

 of them rigidly predetermined to a special cytomorphosis by 

 obscure causes associated with a peculiar provenience of their 

 elements. 



The mesenchyme has long been held to be a complex of various- 

 ly derived elements, which owe their resemblance and solidarity 

 to a common position, not to likeness of origin. That cells free 

 themselves from germ-laj'ers and migrate into the mesostroma 

 between them is well established, and that \-essels make an eai'ly 

 appearance in these strata is indisputable, in the somato-pleure 

 as well as in the splanchno-pleure. But that in the latter situ- 



" 1912, Amer. Jour. Anat., vol. 13. p. 111. Vide .addemlimi. 



" Loc. cit., and Anat. Hefte, Bd. 3.3. 



" Loc. pit. 



" Cf. Ruckert, loc. cit., pp. llSO-81. 



'» Loc. cit., p. 440. 



