388 E, L. CLARK AND E. R. CLARK 



former habit of beating approximately five times during each 

 periodic spasm. However, it did not return completely to its 

 previous mode of functioning for it still continued to contract 

 from 2 to 4 times, independently, at irregular intervals during 

 the period of rest. The observation was continued for an hour 

 after the return of the body movements to their normal periodic 

 rhythm and the lymph heart continued to contract in this same 

 manner (charts 1 and 2: Stage 2). 



Stage 3. Chicks of eight and one-half to nine days (27 to 29 

 mm.) resembled those of Stage 2, except for a slight increase in 

 the independence of the beating lymph heart. An average of 

 six pulsations took place periodically at the time of the body 

 movements, but an isolated beat occurred in nearly every period 

 of rest. Chloretone destroyed the periodicity of the lymph heart 

 beats, which then occurred irregularly, about four or five to a 

 minute. After the recovery from the anesthesia and the return 

 of the body movements, the lymph heart again averaged six 

 beats during each periodic spasm. In addition, it contracted 

 about five times in every period of rest and these isolated beats, 

 as a rule, preceded the period of body movements (charts 1 and 

 2: Stage 3). 



When an embryo of Stage 3 or 4 was anesthetized and the 

 lymph heart punctured by a fine needle, the lymph heart con- 

 tracted six or eight times in succession, with no accompanying 

 tail or body movements. 



Stage 4- In chicks of nine to nine and one-half days (29 to 

 30 mm.) we found that the lymph heart still showed a tendency 

 to beat periodically, but that its rhythm had now become some- 

 what discoordinated from that of the body movements. Although 

 the periodic spasms in this stage were as long as or longer in 

 their dui^ation than those of younger embryos, the number of 

 lymph heart beats occurring at this time was found to be less, 

 an average of three beats instead of six. The number of iso- 

 lated single beats occurring between spasms, however, was in- 

 creased to three or four per interval. The isolated beats, as a 

 rule, preceded the periods of body movements and, immediately 

 after each spasm, there was a period of rest in which neither 



