Responses of Frog Embryos to Tactile Stimulation. 149 



salamanders. This first response is followed by an avoiding-, 

 a double-coil-, an S-reaction and the swimming movement, in 

 order. 



In a number of embryos, the cord was cut at different levels to 

 determine the location in the cord where stimuli are transferred 

 from one side of the body to the other. The results are, briefly, 

 as follows: (1) when the cut passes through the brain, the por- 

 tion anterior to the cut never responds to stimuli, while that 

 posterior to it exhibits the usual series of reactions; (2) when the 

 cut passes through the medulla, the same results are obtained; 

 (3) when the cut passes through the middle of the body at a point 

 just behind the medulla, both parts usually go through the normal 

 series of reactions; (4) when the cut passes just anterior to the 

 tail or (5) through the tail itself, the part of the body anterior to 

 the cut goes through the normal series of responses, while that 

 posterior to it remains negative. From these results it is evident 

 that in the middle of the embryo there is a region about a milli- 

 meter in length which includes the upper part of the spinal cord 

 and the lower part of the medulla, in which the decussations of the 

 primary spinal nerve-paths take place, enabling the transfer of 

 stimuli from one side of the body to the other. 



A large series of experiments on this particular region show that 

 the crossing does not take place at any one easily localizable point, 

 but rather throughout the entire region. When the cord has been 

 cut here, the two portions of the body thus isolated from each 

 other go through the normal series of responses to stimuli inde- 

 pendently of one another, the part in front of the cut usually being 

 somewhat in advance of that behind it. Further, reversal of this 

 region in no way affects the appearance of the responses, nor is it 

 possible to differentiate between the time of appearance of reac- 

 tions in the two extremities of the reversed piece. 



Coghill suggests that the reaction toward the side stimulated, 

 which appears as an aberrant form of response in Amblystoma, may 

 be due to the transmission of the stimuli along the collaterals to 

 the muscles of the same side before the main path to those of the 

 opposite side is fully awakened. That this is actually the case in 

 the frog is apparently demonstrated by the nature of the re- 

 sponses obtained as the embryo enters the second or avoiding- 



