GROWTH OF T3LOOD-VESSELS IN FROG LARVAE 73 



phatics and nerves, and extending between the right and left 

 layers of epiderniis. As the tail grows there is an increase in 

 the number of these cells, and in the size and complexity of their 

 l)rocesses. It is clear that such an increase in the tissue filling 

 up the space formed by a blood-capillary mesh- work, the fine 

 processes of the tissue being in contact with the blood-vessels, 

 would lead to a pushing and pulling on the capillary wall, and 

 it seems (luite safe to infer, as Thoma has done, that the growth 

 in length of vessels is due to the response of the vessel wall to 

 this mechanical tension. 



CHANGE IN THE ANGLE OF BRANCHING 



It was somewhat surprising to find how closely the direction 

 and pattern of the capillaries which persisted resembled, in the 

 latest stage, their pattern in the earliest stage. Thus, bends, 

 present in the capillary at its first formation, although they may 

 diminish, or disappear entirely, may be retained throughout. To 

 a considerable extent, also, the angles between branches re- 

 mained nearly the same as when first formed. It is apparent 

 that the capillaries, once formed, are held fairly rigidly by the 

 surrounding network of fine connective tissue fibrillae. There is, 

 howe\'er, a marked change in the angle of branching, in the case 

 of the arterioles. Here one sees the working out, in a general 

 way, of the laws of branching discovered by Roux, for the smaller 

 the branch, relative to the main stem, the nearer its angle of 

 branching approaches a right angle, and the larger the branch, 

 the more acute — relatively — its angle of branching. Figure 15 

 illustrates the change. On March 18, the two branches — B and 

 C — into which A divides are approximately equal, and the 

 angle of branching of each is nearly 90°. This stage is an early 

 one — the capillaries are newly formed, and are characteristically 

 wide, and the angles are those which happened to form as a 

 result of the direction taken by the new sprouts. March 20, 

 branch B is slightly larger than C, and there is a tendency for 

 its angle of branching to become slightly more acute. March 

 24, April 13, and April 26, however, show a progressive increase 

 in the size of branch C over branch B, and a corresponding dimi- 



