LYMPH-FLOW AND LYMPHATICS, CHICK EMBRYOS 425 



The side pressure in the veins with which they connect is higher 

 than the pressure inside the lymphatics and consequently blood 

 is continually forced out into the extending lymphatics. 



2) The next period (in chicks of 6 to 7 days) in the develop- 

 ing superficial lymphatics is characterized by the establishment 

 of lymph circulation accompanied by the differentiation of 

 definite ducts or channels in the irregular primary plexus. The 

 pressure of the fluid inside the lymphatics first overcomes the 

 pressure in the veins of the neck: the lymph-flow starts in the 

 side plexus and follows a definite course in an anterior direction. 

 Somewhat later the lymph circulation over the pelvis begins and 

 in this region the flow is instigated by the first pulsations of the 

 lymph heart, still in the form of a plexus. The first circulation 

 in the superficial lymphatics is slow and the earliest channels 

 formed are small and tortuous. With the first flow of lymph 

 through the lymph heart plexus, channels also differentiate here. 

 In this stage the blood is gradually washed out of the superficial 

 lymphatic system, first from the side region and later from the 

 lymphatics of the pelvis. 



3) The development of the superficial lymphatics in chicks 

 of 7 to 8 days is characterized by increased pressure in the lym- 

 phatics, stronger pulsations of the lymph heart and a more 

 rapid and steady flow of lymph. Several changes in direction 

 of the lymph-flow occur during this period and the position of the 

 main channels changes with the shifting circulation. The 

 original lymphatic channels enlarge during this stage, and others 

 differentiate from the superficial plexus. The channels through 

 the lymph heart become larger and fewer, and the connections 

 with those intersegmental veins nearest the selected channels 

 are retained and the others are lost. 



4) In chicks of 8 to 9 days, the pressure in the lymphatics 

 is very high. The great increase in the flow of lymph from the 

 allantois and from the deep lymphatics of the body and certain 

 other mechanical factors interfere with the outlet of fluid from 

 the superficial lymph vessels. At this stage the flow is rapid 

 in certain regions and very slow in those places where an obstacle 

 is encountered. Injections show that ducts or channels are pres- 



THE AMERICAN JOURNAI- OF ANATOMY, VOL. 18, NO. 3 



