406 ELEANOR LINTON CLARK 



lymphatic plexus from the vems with which it is connected. 

 These communications between lymphatics and veins can be 

 demonstrated easily by injection in chicks of five days and over. 

 The most favorable place for testing this is in the tail region, 

 where the whole injection can be watched and the ink can be 

 clearly seen as it passes from the lymphatic plexus into the 

 intersegmental coccygeal veins and then runs rapidly anteriorly 

 in the main caudal vein. 



The amount of blood present in the superficial lymphatics can 

 be increased in a living chick by changing the position of the 

 embryo so as to allow the force of gravity to affect various parts 

 of the chick. For example, the posterior lymphatics, which 

 spread out on the surface of the tail and over the posterior 

 border of the pelvis, lie for the most part anterior to the coccygeal 

 veins with which they are connected. By pulling on shreds of 

 the amnion, the tail can be raised so that this part of the chick 

 is uppermost, and the embryo held in this position for some- 

 time. In this case, after the expiration of only a few minutes, 

 more blood enters the lymphatics and the peripheral (lower) 

 portions of the plexus become much redder and more distended. 



Again, the amount of blood in the early lymphatic plexuses 

 can be increased noticeably and quickly by interfering with the 

 blood circulation. When the heart becomes embarrassed from 

 any cause (such as the addition of strong chloretone or too high 

 a temperature of the warm chamber) there occurs a back pul- 

 sation in the veins, which can be observed to best advantage in 

 the large vessels of the allantois and yolk sac. Within a few 

 seconds after the beginning of such a circulatory disturbance, 

 the superficial lymphatics become markedly redder and more 

 congested owing to the increased amount of blood which enters 

 them as a result of this increased venous pressure. 



From these and other observations on living embryos, we came 

 to the conclusion that the normal presence of stagnant blood 

 in the superficial lymphatics, during the primary stage of their 

 development, is dependent upon: 



a. The fact that the early lymphatics form a continuous 

 plexus which is connected at a number of points with veins. 



