378 E. L. CLARK AND E. R. CLARK 



The other type of movements — the active contractions of the 

 embryo — have also been recorded by many observers. Preyer, 

 in an extensive review of the older literature on the subject, 

 mentions that such observations were made by Harvey in 1651, 

 by Beguelin in the middle of the eighteenth century, and by 

 Home in 1822, as well as by von Baer and Remak. Most of 

 these observers, however, did not distinguish between the pas- 

 sive movements of the embryo, caused by the contraction of 

 the amnion, and the active movements of the chick itself. How- 

 ever, Dareste, in 1879, noticed movements in an embryo without 

 an amnion, and thus demonstrated conclusively the independence 

 of this second type of movement. 



Preyer gives an interesting and comprehensive account of these 

 active movements in chick embryos. From the first to the fifth 

 day of incubation he found no embryonic movements, aside from 

 the beating of the heart. Mechanical and electrical stimulation 

 of the embryo produced no effect. On the fifth day, Preyer 

 first observed the active embryonic movements and the amni- 

 otic contractions. The first movements of the embryo, accord- 

 ing to Preyer, are simple in character and consist chiefly in a 

 bending of the back from side to side. The head, tail, and 

 extremities are moved passively at this stage. In successive later 

 stages, the movements become more numerous, violent and com- 

 plicated, as the various parts of the embryo differentiate. At the 

 sixth and seventh da5^ the bending of the body becomes more 

 pronounced, the tail contracts independently, the head nods, 

 and the paddle-like extremities are moved inward and outward, 

 and from the eighth to the eleventh day, the movements con- 

 tinue to increase in strength and variety. Intervals occur at 

 all these stages, during which the embryo does not move. Dur- 

 ing this second period of development — from the fifth to the 

 eleventh day — Preyer found no response of the musculature to 

 mechanical stimulation or to injury, such as amputation of a 

 leg. However he occasionally observed a weak response of cer- 

 tain muscles, following electrical stimulation of the spinal cord, 

 on the ninth and tenth day of incubation. 



