VASCULOGENESIS IN THE CAT 



Of the many problems that engage the attention of students of 

 the vascular system, none is more fundamental, and hardly any 

 more perplexing and confused today, than that of the origin of 

 the simple squamous epithelium (endothelium) which is the com- 

 mon lining of all vascular channels, whether haemal or Ijanphatic. 

 It has been held, and the opinion of late has been zealously propa- 

 gated, that so far as the vessels of the body are concerned, their 

 endothelium arises solely from the already formed endothelium 

 of the extra-embryonic vessels, or if there is any production of 

 endothelium within the body of the embryo, it is for so brief a 

 period and so inconsiderable in amount as to be theoretically neg- 

 Ugible. ' For the supporters of this view Rabl's dictum, "Endothel 

 stammt nur \'on Endothel" is still a satisfactory sunmiary of the 

 facts. A logical sequence of this opinion is the doctrine of the spec- 

 ificity of endothelium, obviously a much broader generalization, 

 which takes endothehum out of the series of mesenchyme deriva- 

 tives and separates it absolutel}' from the blood, should that prove 

 to be of mesodermal origin. Consequent thinkers who support this 

 view ha^•e seen the importance of assigning an entodermal origin 

 to both blood and endothelium. Could this be estabhshed, it would 

 lend a degree of antecedent probability to the doctrine of the 

 angioblast but, strictly speaking, it is not a necessary postulate, 

 for a tissue arising diffusely from the mesenchyme, may as de- 

 velopment proceeds, be confined to definite locahties or even 

 ultimately be restricted to homoplastic j^roliferation. Such a 

 tissue would be specific if it yielded no heterogeneous products, 

 and I feel myself that this latter property enters more largely 



' Minot, Evans, and Sabin in Keibel and Mall, Handbuch d. Entwickelungs- 

 gesch. d. Menschen, Leipzig, 1912. These writers have made a clear and emphatic 

 statement of the doctrines of the Angioblast and of the specificity of endothelium, 

 and presented the evidence which may be adduced in favor of these views very 

 completely. That they have not considered all the available evidence, however, 

 is apparent from their all but complete omission of the results of recent haemo- 

 tology (since 1909), which would indicate the wisdom of some reserve in accepting 

 as final generalizations which neglect such extensive and important data. 



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