440 DAVENPORT HOOKER 
passed through the cord just behind the medulla, the head ex- 
hibited a single reaction toward the side stimulated. The tail 
never showed responses to tactile stimulation until a later time 
than the middle piece. This type of reaction to stimulation on 
the part of the middle piece lasted for several hours. 
The second phase of reaction to stimulation on the part of 
the embryos was marked by the appearance of contractions of 
the middle piece into an are, the concavity of which was away 
from the side stimulated. This is of course a typical avoiding 
reaction. The entire middle piece again responded to stimula- 
tion as a unit. At atime which practically coincided with the 
beginning avoiding reaction in the middle piece, sometimes 
slightly preceding it, but more usually following it, the head 
began to exhibit a single movement toward the side stimulated. 
The myotomes involved were those situated at the extreme 
caudal end of the head region. The tail in the majority of 
cases remained unresponsive to stimulation during this second 
phase of reaction, though in a few cases, just before the begin- 
ning of the third phase it also exhibited a single contraction 
toward the side stimulated. 
The third phase of reaction is marked by the passage of the 
middle piece from the simple avoiding reaction to the double C 
type. Usually at a time which precedes this by not more than 
one to two hours, the head has begun to exhibit an avoiding re- 
action and, almost coincident with its appearance, the tail be- 
gins to respond by a single movement toward the side stimulated. 
The tail portion passes through this period of primary response 
toward the side stimulated very rapidly and soon enters upon a 
period in which it gives a typical avoiding reaction. The char- 
acteristics of the third stage may therefore be said to be as 
follows: (a), head gives an avoiding reaction; (b), middle piece 
exhibits double C reaction; (c), tail passes from a brief stage of 
reaction toward the side stimulated to one in which a typical 
avoiding reaction is shown. 
The fourth stage of reaction to stimulation is characterized 
by the appearance of the S reaction in the middle piece and of the 
double C reaction in both the heac and tail segments. At this 
