THE LATER STAGES OF DEVELOPMENT 159 



laries, but the lumen of a few channels becomes widened, and 

 their walls thickened so that they become the major arteries and 

 veins. Clark and collaborators (1931, etc.) have studied this 

 process in living animals, viz. in the transparent caudal fins 

 of frog larvae, and in specially constructed transparent cells, 

 let into the ear of a rabbit. They found that the fate of each 

 part of the original network depends on the currents which 

 prevail there because of the pressure differences in the system. 

 Where the blood stagnates, the diameter of the vessels de- 

 creases, and finally they disappear. Strong perfusion, on the 

 other hand, causes a widening and thickening of the vessels 

 concerned, leading to their differentiation into major blood 

 vessels. In this way the blood stream itself models the eventual 

 pattern of the blood vessels. 



The gills of larval newts may be taken as a last example. In 

 water with a low oxygen content, these organs are large and 

 strongly branched; the overlying skin is thin, and they are 

 well vascularized. In water with a high oxygen content, on the 

 other hand, where skin respiration can easily take place over 

 the whole body surface, the gills are poorly developed. Here, 

 again, the structure becomes adapted to the functional require- 

 ments under the influence of the function itself. 



