GROWTH OF BLOOD-VESSELS IN FROG LARVAE 59 



duced to its simplest terms, would operate. The square (fig. 

 9 A) represents, diagrammatically, the amount of tissue supplied 

 by the capillary ABC. The large area, supplied by the portion 

 DBF, shown by stippling, causes the amount of interchange 

 through the wall near D and F to become excessive, and new 

 sprouts are sent out which form a new capillary, DEF (fig. 9 B). 

 The relatively diminished area supplied by this new capillary is 

 shown by cross hatching, while the greatly diminished areS, left 

 for DBF is shown by stippling. 



Let us see how this hypothesis accords with the observations 

 of the present study. When capillaries first enter the fin expan- 

 sion there is an extensive, growing non-vascular area, for the 

 fin attains a considerable development before the blood-capil- 

 laries reach it. The formation of new sprouts at this stage is 

 extremely rapid, a rapidity which may easily be accounted for 

 by the excessive amount of interchange involved in the relatively 

 enormous non-vascular area. The new formation takes place 

 in one plane — the sprouts being sent out toward the dorsal and 

 ventral margins of the fin, in the direction to and from which 

 there is clearly the greatest amount of interchange, for the fin^ 

 at this stage, is quite thin. New sprout formation continues, in 

 this plane, until the plexus of capillaries reaches nearly to the 

 fin borders. As the borders are approached, sprout formation 

 diminishes in rapidity, and this may be explained by the rela- 

 tively smaller amount of tissue beyond the furthermost capil- 

 laries. Later, the tissue through the fin increases in all dimen- 

 sions — in thickness, as well as in length and height, the capil- 

 laries increase in length, and a secondary formation of new 

 sprouts takes place in the interstices of the old plexus. Many of 

 the capillaries of this secondary set are in new planes, nearer the 

 surface, especially in the thicker portions of the fin, near the 

 muscle. It is significant that, in this secondary formation, new 

 capillaries may grow out at places in the wall where vessels 

 have been present but have been retracted, at an earlier stage. 

 This secondary formation is best explained by the increase 

 in exchange of substances due to the increase in amount of 

 tissue. 



