430 ELEANOR LINTON CLARK 



In embryos of 8 days, after the valves have formed, blood 

 does not enter the lymphatics in case of a disturbance in the 

 blood circulation, and the movement of lymph may continue for 

 several seconds before it stops altogether. 



In certain embryos of 6 to 7 days, which had been incubated 

 for two or three days at an unusually low temperature, it was 

 found, on opening, that the blood circulation was irregular and 

 the blood vessels greatly congested. In such instances the 

 lymph-flow was almost at a standstill and the superficial lym- 

 phatics were irregular in form, they contained blood, and in cer- 

 tain distended places, resembled the superficial sacs of later 

 stages. 



Jf) The development of valves. This point has already been 

 discussed in connection with the effect of lymph heart pulsations 

 and of a disturbance in the blood circulation and may be sum- 

 marized very briefly here: 



Up to 7| days, no valve is present between veins and lym- 

 phatics. Before the pressure inside the lymphatic plexus be- 

 comes sufficiently strong to overcome that of the communicat- 

 ing veins, blood enters the lymphatics. After the establish- 

 ment of the lymph-flow an abnormal increase in the venous 

 pressure will cause blood to enter the lymphatics. In the case 

 of the tail region the pressure in the veins is normally greater than 

 that of the lymphatics for several days and here a cessation in 

 the lymph heart pulsations permits the entrance of blood into 

 this part of the lymphatic system. With the formation of valves 

 (in chicks of 1\ to 8 days) no more blood enters the lymphatics 

 from the veins. 



In embryos younger than 9 days, there is no valve present 

 between the lymph heart and its superficial lymphatic connections. 

 In consequence each contraction of the lymph heart forces fluid 

 back into the superficial lymphatics as well as into the veins. 

 After the ninth day, a valve is formed and the beating of the 

 lymph heart no longer affects the superficial lymph flow in this 

 way. I have not studied the effect of the development of valves 

 in other parts of the chick lymphatics. 



5) The development of new lymphatics in various regions of 

 the body. As an example of this factor, the rapid development of 



