60 ELIOT R. CLARK 



It would seem difficult to explain the formation of new sprouts 

 — especially the exact location at which they are sent out from 

 the older capillaries — on the basis of the action of specific chemi- 

 cal substances. Such substances, if present, should act equally 

 on all parts of the endothelium, resulting in streams of sprouts, 

 sent out by each endothelial cell affected. We find, however, 

 that excessive sprout formation occurs only in early stages, when 

 the atnount of tissue entirely non-vascular is very great. Later, 

 when the tissue has received its primary supply, and when new 

 sprouts are clearly associated with the general enlargement of 

 the organs, the formation of new sprouts occurs in a much more 

 orderly fashion, a single new sprout here, another there, a condi- 

 tion which seems much better explained by the hypothesis that 

 the new formation is due to increase in interchange of substances 

 beyond a certain point, than by supposing the presence of specific 

 chemical substances. 



On more general grounds, also, the proposed hypothesis seems 

 the most plausible. It has been brought out especially by Roux 

 that the growth, maintenance and atrophy of tissues is to a 

 considerable extent regulated by the extent of their performance 

 of certain functions in the body as a whole. Increased or di- 

 minished function results in increased or diminished growth. 

 The endothelium of blood capillaries functions as a membrane 

 through which substances pass to and fro between the lumen and 

 the fluid outside. It would be in harmony with Roux' general 

 conclusions, if it were found that the new growth, maintenance 

 and atrophy of capillaries is regulated by the intensity of this 

 passage of substances through their endothelium. 



To be sure, it is impossible to go with certainty beyond the 

 conclusions of Mall that, ''with the new formation of tissue new 

 blood-vessels may grow into it"— that it is "the growth of the 

 tissue which leads the way," and that ''into this new-formed 

 tissue the capillaries grow." Nevertheless the proposed hypoth- 

 esis as to the precise formative stimulus seems to the author 

 to be more in accordance with the facts than the other hypotheses 

 which have been suggested. 



