GROWTH OF BLOOD-VESSELS IN FROG LARVAE 75 



the response of the tissue to hydrodynamical factors, and that 

 the angle formed is always the one by which the minimum of 

 friction is permitted. 



The same problem is discussed by Thoma, ('11, p. 26) who 

 agrees in the main, with Roux' findings, but considers that the 

 blood-vessel, in assuming this shape, is merely responding to the 

 rate of blood-flow, according to his first histomechanical law. 



It would seem that Thoma's explanation (if amount of flow is 

 substituted for rate) is sufficient, for, if fluid flowing through 

 any tube tends to leave at a greater and greater angle the smaller 

 the opening, then, if the blood vessel did not correspond with 

 the direction naturally taken by the fluid stream, part of its 

 wall would be subjected to the action of a greater flow of blood, 

 than other parts, and would enlarge, while other parts would 

 retract from the opposite cause. By this, the branch would be, 

 remodelled until its angle of branching represented that in which 

 the blood flowed in even amounts over all parts of the wall. 



My observations on the angle of branching add nothing new 

 to the results obtained by Roux, except that they represent 

 studies on the same vessels at different stages, and give a picture 

 of the actual changes in shape going on, hand in hand with the 

 changes in the relative sizes of the branches, together with an 

 approximate record of the relative amount of blood flow. 



STUDY OF VESSELS IN TAD-POLES WITHOUT HEARTS 



Since it has been found that a considerable development takes 

 place in embryos deprived of a circulation by eliminating the 

 heart-beat, it would seem, at first sight, that an opportunity 

 offered itself here to test certain of the hypotheses dealt with in 

 this paper — to find out whether any, and, if any, how much de- 

 velopment of arteries, veins and capillaries takes place — particu- 

 larly in the portion of the tad-pole's tail studied. 



Brief reference has been made to such studies, and they will 

 now be referred to more fully. 



Roux ('95, p. 83) refers to a picture given by Dareste ('77, PI. 

 VII, fig. 6) of a chick embryo in which the embryo proper failed 

 to develop. In commenting, Roux says: 



