432 ELEANOR LIKTON CLARK 



SUMMARY OF RESULTS 



The principal results of the present study may be summarized 

 as follows : 



1. The primary superficial lymphatics of chick embryos 

 form a rapidly growing, frequently anastomosing capillary net- 

 work. This primitive plexus maintains numerous open con- 

 nections with the venous system in certain places. For over 24 

 hours the pressure in these earliest lymphatics remains less than 

 the side pressure in the connecting veins and, consequently, there 

 is no lymph-flow in the early plexus. Instead, it contains blood 

 which backs up into it from the communicating veins. 



2. The pressure of the fluid in the lymphatics gradually 

 increases and finally overcomes that of the veins. The first 

 lymph-flow which is then established is feeble and easily dis- 

 turbed. The lymph-flow gradually becomes more rapid and 

 steady, but its course is easily altered by various mechanical 

 factors. A day later the pressure in the superficial lymphatics 

 has increased still more, while, for various reasons, the out-flow 

 into the veins is interfered with. At certain points two conflicting 

 pressures are present and here the lymph-flow becomes sluggish. 



3. The endothelium of these early lymphatics responds to the 

 passage of fluid over its interior by the differentiation of definite 

 ducts or channels out of the indifferent primitive net-work. 

 With the increased flow of lymph these channels enlarge and more 

 channels form. That the formation of such channels is due to 

 the lymph-flow and to mechanical factors rather than to arbi- 

 trary predetermination is evident from the frequent variations 

 which occur in the position of the main ducts in chicks of the 

 same stage. The endothehal wall of the early lymphatics also 

 responds to the increased pressure caused by interference with 

 the lymph-flow and the damming back of fluid, by expanding 

 to form sac-like enlargements. The size which these sacs may 

 attain is influenced to some extent by the looseness of the sur- 

 rounding tissue. 



