(iltOWTH OF BLOOD-VESSELS IN FROG LARVAE 69 



length of the periods of no circulation before retrogression takes 

 ])lacc. I have no entirely satisfactory explanation to offer to 

 account for the stoppage of cinuilation in these branches. It is 

 possible that the increasing thickness and elasticity of the arteri- 

 ole causes a constriction a])()ut the opening of the branch, until 

 it becomes so small that a blood cell cannot pass through. An- 

 other possil^ility which has suggested itself is that the narrowing 

 is due to a suction on the branch caused by the rapid passage of 

 fluitl past an opening into a branch going off at a right angle. 

 A\'hate\er the explanation, however, the important fact remains 

 that the retrogression of these branches is associated with a 

 diminution in the total amount of blood which passes through 

 them. 



There remain vessels in which the blood-flow diminishes and 

 the lumen decreases, with resultant retraction, for which none of 

 the factors thus far suggested seems to offer a satisfactory explana- 

 tion. It is .quite a striking fact that, in most new capillaries 

 the blood flow is at first slow — usually slower than in older 

 capillaries — that the circulation through the newer capillaries in- 

 creases while that through some of the older ones diminishes. 

 It would seem that here, as in the case of the formation of new 

 sprouts, a regulating factor must be looked for in the rate of 

 interchange of substances through the wall. A glance at the dia- 

 gram (fig. 9) will illustrate the significance of this suggestion. 

 While the portion of the capillary DBF, in figure 9 A, is so placed 

 that it forms the medium of interchange for the large stippled 

 area; in figure 9 B, the new capillary DEF, which has developed 

 peripherally, has taken over the greater portion of this area, 

 leaving only the small stippled area for capillary DBF. It 

 seems logical to suppose that, if the area . supplied by such a 

 capillary is sufficiently reduced so that the amount of interchange 

 through the wall falls below a certain point, the capillary wdll 

 diminish in caliber and eventually retract. 



The capillary is concerned chiefly with this interchange of 

 substance, and it is difficult to escape the conclusion that the 

 growth processes of endothelium are regulated by it. Were 

 they not, it seems impossible to conceive of how an organ becomes 



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



