LYMPH-FLOW AND LYMPHATICS, CHICK EMBRYOS 401 



further. On several occasions, he saw a lymphatic sprout grow 

 out to a group of red blood cells, which had been extruded into 

 the surrounding mesenchyme, and take them in, one by one. 

 From these and other observations he advanced the hypothesis 

 that the accumulation of substances (probably products of cell 

 metabolism) and the passage of such substances through the 

 wall of the lymphatic, stimulates the new formation and further 

 growth of lymphatic capillaries, that the greater the accumu- 

 lation of these substances, the longer and more persistent the 

 growth processes, and that it is the varying formation of such 

 substances which regulates the peripheral growth of the lymphatic 

 capillaries. 



Evans^ made the observation that new lymphatic capillaries 

 grow into tumors of connective tissue origin but that they do 

 not invade those of epithelial origin, and concluded that the 

 growing lymphatics respond to specific chemiotactive influences 

 in the surrounding tissue. That the substances which act as 

 stimuli of lymphatic growth differ from those influencing the 

 growth of blood capillaries is shown by the fact that the latter 

 type of tumor, described by Evans (the one in which no lymphatics 

 were present) contained an abundant supply of newly formed 

 blood vessels. 



Knower,^ from a study of injections of the lymphatics in living 

 amphibian embryos, has concluded that the differentiation of the 

 earliest lymphatics is initiated by the accumulation of various 

 metabolic substances in the tissue spaces and has suggested a 

 correlation between the rapid growth of the first lymphatics 

 and the early function of the pronephros. 



In the present study, an attempt has been made to continue 

 this line of investigation of the factors which regulate the growth 

 of early lymphatics by studying simultaneously a few of the 



* H. M. Evans, On the occurrence of newly-formed lymphatic vessels in malig- 

 nant growths. Johns Hopkins Hosp. Bull., vol. 19, no. 209, 1908. Ueber das 

 Verhalten der Lymphgefasse bei experimentell erzeugter Peritonealcarcinose 

 der Maus. Tubingen, 1912. 



^ H. McE. Knower; A comparative study of the embryonic blood vessels and 

 lymphatics in Amphibia. Proc. Amer. Assoc. Anat., Anat. Rec, vol. 8, no. 2, 

 1914. 



