GliOWTH OF BLOOD-VESSELS IN FROG LARVAE 83 



by the irregular indifferent plexus with no tendency toward 

 transformation into arterioles, venules and capillaries, showing 

 little more than the ability to send out and withdraw sprouts, 

 is in marked contrast to the picture presented by the vessels in 

 the same region, in tad-poles with a healthy circulation. 



b. In normally developing tad-poles, the establishment of 

 circulation brings into play, on the endothelium, the distending 

 action of the blood-pressure, the mechanical friction of the mov- 

 ing blood stream, the mechanical (and possibly chemical) action 

 produced by the passage of substances through the endothelial 

 wall, in the interchange between the blood and the tissue fluid, 

 and the pull and push of the enlarging and shifting outside tis- 

 sues and organs. These studies, which have been principally 

 confined to this stage, indicate, in general agreement with Thoma, 

 Roux, Mall and Evans, that the new factors come to play a pre- 

 dominant part in the regulation of the growth of the vascular 

 system, a part so important that the vascular endothelium may 

 be said to depend for its growth on its response to the action of 

 these forces. 



The morphological changes are as follows: 



The blood-vascular system extends by the well-known method 

 of sprout formation. Blood-vascular endothelium has an affinity 

 for other blood-vascular endothelium, cytotropism, (Roux), and 

 avoids cells of other types of tissue, so that connections form be- 

 tween one sprout and another, or between a sprout and a fully 

 formed capillary, while no connections form betw^een a sprout 

 and foreign cells. Thus the vascular system growls as a con- 

 tinuous network. 



A sprout once formed may have one of several fates; it may 

 enlarge to form part of an arteriole or venule, which may become 

 an artery or vein, or it may remain a capillary, or it may retro- 

 gress, losing its lumen, separating in the middle, and retracting 

 into the capillaries with which it is connected at its two ends. 



Changes take place in the angle of branching, and vessels in- 

 crease in length. 



The mechanical conditions which regulate these morphologi- 

 cal changes are the following : 



