VASCULOGENESIS IN THE CAT 7 



return it to the heart. From this period throughout life, such 

 continuous vessels are present and functionally active. In the 

 adult body it has long been customary to inject them for conven- 

 ience in demonstrating their course. In the foetus also, such 

 procedures are familiar and have been held to demonstrate the 

 plan of organization of the circulatory channels at the period of 

 examination. Now it is obvious that the injection at earlier 

 periods, even in young embryos, is a fact of the same order, how- 

 ever admirable the technique or delicate the procedure it sim- 

 ply demonstrates continuous channels, which it may be presumed 

 are functionally active. It reveals the extent and conformation 

 of the formed and functional vascular system and enables the 

 observer to follow its successive stages from the period at which 

 injection first becomes practicable. In such studies, however, 

 injection has no monopoly, but it shares its data with the method 

 of direct observation of living tissue when the object is small and 

 transparent, and with the exact but laborious method of recon- 

 struction. On the other hand, it is idle to pretend that the in- 

 jection method throws any light on areas beyond the limit of the 

 actual injection, or affords the slightest presumptive evidence 

 of their histological condition, in particular, as regards the- pres- 

 ence or absence of small discrete vesicles of endothelium. 



The dilemma is simple; we may recognize or refuse to recog- 

 nize the uninjectible periphery. If we recognize it, the method 

 of injection becomes not only partial and incomplete, but subor- 

 dinate to the methods which reveal all of its findings and in addi- 

 tion enlarge our field of observation. It ceases to be a critical 

 method. If we refuse to recognize the periphery beyond our 

 injection, we have begged the question at issue, our vessels are 

 growing not only in a foreign tissue, as the angioblast doctrine 

 believes, but in a theoretical vacuum and the only possible source 

 of increment is the proliferation of their own elements. We 

 reach the only conclusion the exclusive study of organized ves- 

 sels by any method would allow, the formation of the new out of 

 the old by sprouting. A comparison may perhaps serve to il- 

 lustrate the inconvenience of taking continuity of lumen as the 

 critical index of structure. "Had the metanephros chanced to 



