276 SELIG HECHT 
The fatigue of the muscles which cause the contraction is also 
not at the bottom of the lack of response. When an animal 
failed completely to respond to vibrations, a different stimulus, 
such as the application of heat or chemicals, at once resulted in 
a reaction. Also, as Kinoshita (11) showed for Ciona, the 
cessation of response when a certain spot was regularly stimulated 
is concerned with that spot only, because immediate irritation of 
a different locality at once called forth a reaction. There‘ore 
the phenomena are not concerned with the exhaustion of the 
muscle layer. 
All these occurrences in Ascidia are best explained on the 
assumption that the cessation of reaction is due to the fatigue 
of the receptor. This accounts for the localization of the effect, 
and for the response to a different quality of stimulation. More- 
over, it makes a record like that of figure 2 intelligible. In the 
first interval during which the animal did not respond, the sense 
organ recovered to the extent of being efficient for quite a while. 
But as the process of fatigue was continued, recovery occurred 
only to the extent of enabling the sense organ to receive a single 
stimulus now and then. 
The further history of an animal in such a condition is in- 
structive. If it be allowed to remain undisturbed for a few min- 
utes, almost complete recovery may occur. For example, the 
animal whose record is given in figure 2 was not stimulated for 
five minutes after the point d. During this time there occurred 
a spontaneous contraction (e) the nature of which has already 
been discussed (Hecht ’17).6 At the point f stimulation was 
again begun. It will be seen that the amplitudes of the con- 
tractions are comparable to those produced at the beginning of 
the record. After about ten minutes the experiment was dis- 
continued because of the sheer exhaustion of the observer. 
These experiments leave no doubt that fatigue is a factor of 
major importance in the effects of repeated stimulation. When 
5 The appearance of the usual rhythmic contractions in fatigue experiments 
shows also that we are not dealing here with the exhaustion of the muscles, but 
rather with the fatigue of the sense organs which furnish the Anlass for the muscu- 
lar contraction. 
