380 E. L. CLAKK AND E. R. CLARK 



One very striking characteristic of the active movements of 

 chicks during the period of their development from four to four- 

 teen days, is their periodicity. Tlie movements occur in defi- 

 nite groups or spasms followed by an intermission or period of 

 rest, after which the movements recur. This periodicity was 

 not noted by Preyer or by earlier observers, although they record 

 the fact that the active movements were not continuous. 



While the active movements of the embryo can be observed 

 with the amnion intact and have been studied through this mem- 

 brane, they are more easily observed when it is cut across and 

 drawn aside. In observing the periodic movements, we kept 

 records of the minute and second at which each spasm com- 

 menced, its duration, and the interval from the beginning of one 

 spasm to the beginning of the next. We found that the duration 

 of the periodic movements and the periods of rest varied in 

 embryos of different ages. For example, in a chick of six and 

 one-half days, the duration of the periodic spasms averaged 13 

 seconds and the period of rest 39 seconds, while in an embryo of 

 nine days, the movements were found to continue for 28.5 sec- 

 onds and to be followed by an average period of 54.5 seconds, in 

 which the chick did not move. In all the stages studied, one or 

 two movements of the tail, a slight twitch of the leg or wing, or 

 a single jerk of the whole body occasionally occurred, apart from 

 the periodic spasms. The interval from such an isolated move- 

 ment until the beginning of the next was always shorter than 

 the interval between the true periodic spasms. 



We also made some observations of the reaction of embryos 

 to changes in temperature. When the temperature of the warm 

 chamber was raised until a small thermometer placed near the 

 embryo registered 41°, the active movements ceased, after a 

 few minutes, and the chick remained motionless until the tem- 

 perature was lowered to 38 or 39°, when the periodic movements 

 were resumed. We also found, as Dareste had observed in the 

 case of a chick without an amnion, that the muscular move- 

 ments ceased when the temperature was lowered to 35° or under 

 and began again when the temperature was raised. When the 

 amnion was left intact, the response of the body movements to 



