GROWTH OF BLOOD-VESSELS IN FROG LARVAE 61 



INCREASE IN SIZE OF CAPILLARIES TO FORM ARTERIOLES AND 



VENULES 



It is obvious, in looking over the series of changes which take 

 place in the capillaries, that, while some remain capillaries or 

 are retracted, others gradually increase in size to form arterioles 

 and venules. A study of the position of those which increase in 

 size, shows that the increase takes place in the capillaries or 

 parts of capillaries which are so placed that they form vessels for 

 the suppty or drainage of larger and larger capillary areas, so 

 that their endothelium is subjected to the action of the passage 

 of an increasing amount of blood. It would seem, then, that 

 the conclusion is justified that increase in the size of the lumen 

 of a capillary is regulated by the amount of blood flow. This is 

 somewhat similar to the conclusion of Thoma, expressed in his 

 first histomechanical law, that the size of the lumen depends 

 upon the rate of the blood flow, at a minute distance from the 

 wall. There is, however, this difference, that according to 

 Thoma the rate of flow is the determining factor, while the present 

 studies indicate that it is the total amount rather than the rate. 



The capillaries in early stages — that is, during their early ex- 

 tension into the fin — are markedly wider than a few days later. 

 At the same time the rate of flow in all vessels, at the earlier 

 stages, is decidedly less than later — so that, coincidently with 

 the increase in rate, there is, at this stage, a general diminution 

 in the size of the lumen of all vessels. Moreover, in later stages, 

 new capillaries are, for a time, relatively wide, with a slow circu- 

 lation, and become narrower as the rate of circulation through 

 them increases. Again, many instances may be seen, in any 

 growing tad-pole's tail, of vessels remaining the same size or 

 even diminishing in size until the lumen is obliterated, through 

 which the rate of blood flow is relatively rapid, but, because of 

 frequent complete stoppage of the flow, with the total amount 

 very small, while, side by side with them, new capillaries, with 

 a decidedly slower circulation, though they may diminish slightly 

 in size, are not obliterated. 



Another fact which must be referred to here is the well-known 

 one that veins, in general, have larger diameters than arteries, 



