CoGHiLL, T Jie Reactioji to Tactile Stnniili. 99 



only; (2) the "JS" movement or reaction, which is a bending of the 

 more cephalic and the more caudal parts of the body in opposite 

 directions, giving the form of the letter S. 



The flexure may occur in several varieties. It may be a "head 

 flexure," which effects a movement of the head only; a "pectoral 

 flexure," which affects slightly more of the trunk than the head 

 Hexure does ) a "mid-trunk flexure," which is effected by the muscles 

 of the middle portion of the trunk only; a "general flexure," which 

 involves the bending of the whole trunk. In the mid-trunk or 

 pectoral flexure the parts cephalad and caudad of the flexed part may 

 assume positions parallel to each other, in the form of the letter U. 

 This may be designated as the "U" reaction. The general flexure 

 may be extended till the body assumes more or less a coiled con- 

 dition. This movement may be termed the "coiled reaction." 



The various forms of the flexure are not to be considered as essen- 

 tially distinct, for, with possibly the exception of the U reaction, 

 they develop gradually one into the other in the order mentioned. 

 ]S[evertheless, the distinctions are useful for descriptive purposes. 



The first member of this series to appear in the course of develop- 

 ment of the embryo is the head flexure ; the next is the pectoral 

 flexure, and, as the embryo advances in age, the flexure extends farther 

 caudad untill it involves the entire trunk in a general flexure, and, 

 finally, in a coiled reaction. In ontogeny, then, the flexure develops 

 cephalo-caudad. This is true for responses to stimulation on the 

 tail bud as well as for responses to stimulation on the head. 



In the development of any particular flexure, pectoral, general or 

 coiled, the same progression cephalo-caudad is observed. If the n. 

 trigeminus or n. vagus is stimulated by a touch, the normal reaction 

 is a head flexure, and, if the embryo is sufficiently advanced in age, 

 this flexure progresses caudad until the whole trunk is involved.. 

 In like manner, if the touch is upon the tail bud, the response begins 

 in the head region and progresses caudad. The physiological de- 

 velopment of a flexure, then, is correlated with its ontogenetic develop- 

 ment. 



JSTow, so far as my observations go, the S reaction never appears 

 until the embryo is capable of executing an extended general flexure, 



