GROWTH OF BLOOD-VESSELS IN FROG LARVAE 63 



obliterated. (This will be taken up more fully later.) Thoma 

 has made similar observations on ligated vessels — finding that, in 

 spite of the mechanical distention due to blood-pressure, the 

 lumen of ligated vessels becomes reduced to zero between the 

 point where the last branch is given off and the point of ligature. 

 We are, then, in agreement with Thoma, on this point, that 

 the endothelium responds to the action of a moving fluid. To 

 us, however, it appears that Thoma' s claim that it is the rate of 

 flow is not justified. Rather, it appears that it is the amount of 

 blood flow through a vessel which determines the size of its 

 lumen. To be sure the factor of rate of flow, as well as the closely 

 related factor of blood pressure, cannot be disregarded. To a 

 certain extent, however, their action appears to be the opposite 

 of that claimed by Thoma, for with increased rate there may be 

 a diminution in size. 



THE REGRESSION OF VESSELS 



It has already been mentioned that in the growth of blood 

 vessels many capillaries and parts of capillaries disappear. This 

 process was referred to briefly in a former paper (2), '09 and will 

 now be examined more in detail. On the series of records shown, 

 there are many instances of the disappearance of vessels, in some 

 cases of vessels which had never attained a circulation, in others 

 of vessels which had had an active circulation. 



First let us see what are the morphological changes which 

 take place in the 'disappearance' of a vessel. The process has 

 been watched carefully many times, and two sets of records are 

 reproduced to show the details (figs. 10 and 11). The first 

 change to be noted is a narrowing of the lumen. Next there 

 appears an interruption of the lumen by the formation of a 

 solid portion. The solid part may start near the middle of the 

 capillary or nearer one end, and it gradually increases in extent 

 toward the two ends. As it increases, the solid portion becomes 

 narrower and narrower, until a varying amount of the former 

 capillary is represented by only a fine thread. This thread be- 

 comes thinner until it is barely visible, and eventually disappears, 

 leaving the two ends forming blind-ending projections from the 



