422 ELIOT R. CLARK AND ELEANOR LINTON CLARK 



indopoiideiit tissues. Hence, in studying problems dealing 

 with the growth of lymphatics and blood vessels in the tad- 

 pole's tail, the question of the primary differentiation of endo- 

 thelium from mesenchyme is not involved. But the question 

 of the nature and reactive powers of the lymphatic capillary, 

 its relation to the tissue spaces, and its role in the process of 

 absorption can be investigated to advantage in living tad- 

 poles. The importance of some of these problems his been 

 emphasized by Miss Sabin (26) in a recent summary of the in- 

 vestigations on the growth of the lymphatic system. 



Our interest in the present problem started with the observa- 

 tion by one of the authors (3), published in 1909, that lymphatic 

 endothelium, in the tail-fin of the tadpole, reacts to the pres- 

 ence in the tissue spaces of red blood cells which had been ex- 

 truded by pressure from the blood capillaries. In such cases, 

 lymphatics grew out to the blood cells and actively engulfed 

 them. Since the morphological changes which the lymphatic 

 underwent, in reaching blood cells at a distance, were similar 

 to the usual growth changes of lymphatic capillaries, it was sug- 

 gested that the normal growth of lymphatic endothelium, after 

 its primary differentiation, represents a response to specific 

 substances outside, which are usually invisible. 



That specific external substances may be a growth regulating 

 factor for lymphatic endothelium, was also suggested by Evans 

 (8, 9), w^ho found that lymphatics fail to grow into tumors of 

 epithelial origin, while they do invade tumors of connective 

 tissue origin. He proposed the hypothesis that, in the first 

 case, there are present angio-repellant substances, and, in the 

 second case, angio-tactic substances. This hypothesis has not 

 been tested. 



It occurred to us that it might be possible, by injecting cer- 

 tain substances into the transparent fin expansion of the tad- 

 pole's tail, to gradually obtain information as to whether the 

 lymphatic endothelium is influenced by them and, if so, by what 

 types of substances. The selection of substances which presum- 

 ably might have a specific attraction for lymphatics was pre- 

 ceded by injections of an inert substance. The results of in- 



