428 ELEANOR LINTON CLARK 



greater until in an embryo of 7 to 7^ days the direction of the 

 entire lymph-flow is posterior, through the lymph heart, into the 

 veins of the tail. At this stage the function of the lymph heart 

 is still involved with the muscular contractions and can be 

 paralyzed by chloretone. In such a case (provided the blood 

 circulation is undisturbed) after a few moments of uncertainty, 

 the lymph-flow is completely reversed and granules which had 

 previously all been travelling posteriorly from the scapular 

 plexus, over the side and pelvis to the lymph heart, now follow 

 the same paths anteriorly and flow into the deep jugular plexus 

 through the connection anterior to the shoulder. This experi- 

 ment has been repeated many times. In such cases the lymph 

 heart can be stimulated to resume its function by direct puncture 

 of its muscular wall. WTien this is done or when the lymph 

 heart begins spontaneously to pulsate, after recovery from the 

 anaesthesia, circulation is again reversed and the lymph-flow for 

 the entire surface follows its original posterior course into the 

 lymph heart. 



These observations show that the pulsation of the lymph heart 

 is an important factor in overcoming the high pressure of the 

 tail veins, that it instigates and maintains the flow in the lym- 

 phatics of the pelvis, and that, until valves have formed at the 

 venous connections, it prevents blood from backing up from the 

 veins. The lymph heart exerts an important influence on the 

 whole superficial lymph flow for a period of about 24 hours. 



Although the fact that the superficial circulation can be re- 

 versed during this stage, shows that the contraction of the 

 lymph heart is at no time indispensable to the flow of lymph in 

 the superficial vessels, its absence would probably affect the 

 lymph-flow unfavorably. This was shown by one embryo which 

 appeared to be normal except for a stunted tail. A diminutive 

 lymph heart about one-third the normal size, which beat fre- 

 quently but feebly, was associated with this deformity. The 

 embryo was oedematous and the lymphatics of the pelvis which 

 drained into the lymph heart were large and distended. Al- 

 though the embryo in other respects resembled a 7-day chick 

 (the stage in which the superficial lymph-flow is rapid and 



