No. 390.] THE ECTODERM OF HYDRA VIRIDIS. 507 
a continued application of the stimulus for a longer time before 
the response takes place. These latter tend to group themselves 
about a mode of thirty-five (seconds). Between these two almost 
no cases were found. 
I cannot explain the loss of the ectoderm. It could not be 
due to heat, for the alum cell effectually cut out that element, 
as was shown by placing the bulb of a delicate thermometer in 
80-44 
754 75-4 
70-7 704 
65- 65- 
60- 60- 
eT 55- 
50-4 504 
45- 45 
40- 40 
a4 35- 
30- 30- 
254 25- 
20- 
15 15-4 
i k 
5 5 
I 234 s 6 7 8 QI01I 5 1015 20 25 30 35 40 
Fic. 1. Fic. 
Fic. 1. — Curve of the first response. The ordinates represent time (in sarong and abscissas 
represent individuals. It is to be noted that the individuals fall in two classes with one 
mode at ten ree the other at thirty-five se 
Fic. 2.— Loss of ectoderm. scsi wile, _~ ea of the individuals about two 
modes; one at five, th Ordinates = time in minutes, Abscissas = 
number of individuals. 
the aquarium containing the hydra. There was no change what- 
ever, and the water remained constantly at the temperature of 
the room, which ranged from 15° to 18° C. It could not have 
been due to escaped currents of electricity, for insulation was 
perfect. Neither could it have been the effect of the Roentgen 
rays, although such are found in the arc-light. The distance of 
the object from the arc was forty-two inches, and the small 
amount of those rays generated by the arc-lamp could not have 
had, it seems, any effect at so great a distance, inasmuch as 
they were not focused or controlled by optical appliances. This 
dropping off of the hypodermis, however, resembles in many ways 
the blistering effects produced upon the skin by Roentgen rays. 
