24 VASCULOGENESIS IN THE CAT 



employed. I have ah-eady expressed a doubt that this was the 

 case and have suggested that different principles of interpretation 

 were at the root of the difficulty, and I believe that the fault lies 

 with the upholders of the doctrine of specificity, in their too great 

 confidence in the uniformity and simplicity of ontogeny, in their 

 failure to distinguish between genesis and growth, and in the pre- 

 maturity of theii' assertion that endothelium is specific, before 

 they have canvassed its potentialities and seriously studied its 

 regressive phases. 



The tendency to minimize the period of development, while 

 the natural outcome of reliance upon injections which reveal 

 only formed endothelium and continuity of lumina, has given 

 the theory of specificity a strangely preformist character. Con- 

 sistently carried out, it would reduce the history of endothelium 

 to the expansion and ramification through the body of a single 

 anlage, produced at a distance from the embryo in the mesoderm 

 or from the entoderm of the yolk-sac. The innumerable separate 

 anlages here, and their gradual concrescence are supported by a 

 weight of evidence unpossible to set aside, but the facts seem easier 

 to be borne when this multiplicity is veiled as far as possible by 

 using the term 'anlage' in the singular number and the collec- 

 tive sense. Yet even this 'anlage' proves unable to account for 

 all the vessels of the embryo by its expansion, though Evans feels 

 that most of those in the splanchno-pleure are formed bj^ sprout- 

 ing. Exception must of course be made of the early ones in the 

 area opaca, the 'anlage' itself, and those in the area pellucida 

 which McWhorter and Whipisle have shown are formed by 

 concrescence. 



Still there are further difficulties, certain vessels of the embryo 

 arise in situ — the aorta in the head, the first arch, parts of the 

 dorsal aorta, perhaps the heart ; it is possible to attach too much 

 importance to the early connection of the caudal portion of the 

 aorta with the splanchnopleuric plexus (in chicks of 20 somites, 

 Evans), and in the face of very general evidence of in situ origin 

 it can only be maintained that any vessels which have not been 

 individually proved to arise in loco are derived by ingrowth from 

 the 'anlage,' with the lumen of which their lumina are shown by 



