72 ELIOT R. CLARK 



THE INCREASE IN THE LENGTH OF VESSELS 



With the general growth expansion of the tissue, the question 

 arises as to what effect this has on the blood vessels present in 

 the midst of the organ. In figures 13 and 14 there have been super- 

 imposed capillaries and vessels which were present at the earliest 

 and at the latest records. In one case (fig. 13), they are both 

 drawn at the same degree of enlargement. In the other (fig. 14) 

 the drawing of the older stage has been reduced, by means of the 

 pantograph, enough so that the two sets of vessels have the same 

 dorso-ventral dimensions. Measurements show that the in- 

 crease has been greater in the antero-posterior than in the dorso- 

 ventral direction. The increase in the dorso-ventral measure- 

 ment of the fin expansion has been in the proportion of 1 to 2.22; 

 while in the antero-posterior measurement (the total length of he 

 tadpole) the increase has been in the ratio of 1 to 2.73. Thus 

 the ratio of the dorso-ventral to the antero-posterior increase is 

 about 7: 10. Almost the same proportion is found to exist be- 

 tween the measurements of corresponding parts of the capillary 

 plexus at the same two stages. Thus the ratios obtained are, 

 for the dorso-ventral increase: 1. to 2.2, for the antero-posterior 

 increase: 1. to 2.9. It is possible that the agreement between 

 the two sets of antero-posterior measurements would be even 

 closer, had the measurement been made of the increase in length 

 of the tail, instead of the increase in the length of the entire 

 larva, including the head. It is obvious that the growth of the 

 tissue has caused a proportionate growth in the length of the 

 blood capillaries, and the size of the capillary mesh-work. 



There would seem to be but one possibility as to the factors 

 responsible for this increase in length of vessels, namely, the one 

 proposed by Thoma ('11), as his second histomechanical law, that 

 increase in length of vessels results from a tension exerted in a 

 longitudinal direction on the vessel wall, by the surrounding tis- 

 sue. In the tail of the frog larva, the space between the blood 

 capillaries is occupied mainly by branched mesenchyme cells, 

 from which fine fibrillae in great abundance are given off in all 

 direction, surrounding and supporting the blood-capillaries, lym 



