(iROWTH OF BLOOD-VESSELS IN FROG LARVAE 85 



]:)()scd as an hypothesis, because it seems to fit the facts better 

 than any other. According to this hypothesis, the endothehum 

 of blood-capillaries responds to the passage through it of various 

 substances, in the interchange which takes place between the 

 blood and the outside tissues. To an increase, beyond a certain 

 maximum, the endothelium is thought to react by sending out a 

 sprout; to a diminution, beyond a certain minimum, in a capil- 

 lary which is not so placed as to form part of an arteriole or 

 venule, the capillary is thought to react by narrowing its lumen; 

 while for the maintenance of a capillary, a certain intensity of 

 interchange is thought to be necessary. 



The formulation of this hypothesis, which agrees, in general 

 with Roux' conception, is merely carrying the explanation for 

 the new formation of capillaries a step further than Mall, who 

 recognized, as did Thoma, that the ultimate cause for new growth 

 of capillaries lies in the growth and metabolism of outside tissues. 

 It seems to fit more facts than the suggestion of Loeb and Evans 

 that the cause lies in the action of specific substances outside 

 the capillaries, or Thoma's hypothesis that it results from in- 

 crease in capillary blood-pressure. It needs, however, further 

 proof, before it can be accepted as a ''law of growth." 



The changes in the angle of branching, which were observed, 

 represent a response to the relative amount of blood-flow through 

 the branch, as compared with the main stem. If a branch re- 

 mains or becomes relatively large, as compared with the stem 

 vessel, the angle between the two approaches 0°, if relatively 

 small, 90°. This is, in general, in agreement with the studies of 

 W. Roux who has made elaborate mathematical estimations of 

 the angle of branching, and finds that the relation is so pre- 

 cise that it can be expressed within limits, as a mathematical 

 formula. 



The growth in the length of vessels goes hand in hand with 

 the increase in outside tissue, and is clearly, as Thoma has ex- 

 pressed in his second histomechanical law, brought about by the 

 reaction of the vessel to the mechanical pull, exerted in a longi- 

 tudinal direction on the vessel. 



Thoma's third law, according to which the thickness of the 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 23, NO. 1 



