404 THE AMERICAN NATURALIST. [Vor. XXXIV. 
possible influence of temperature upon the maturity of the 
medusz, and suggested also additional experiments. Artificial 
change of temperature by means of heat or ice had a very 
noticeable effect, both upon the activity of the medusz and 
the cleavage of the eggs, the reduction of temperature by a 
few degrees materially retarding the rate of cleavage, while 
raising it a corresponding amount proportionately accelerated 
these phenomena. This was likewise evident in the develop- 
ment and activity of the larvae. 
3. Artificial Division of the Eggs.— During the earlier 
phases of segmentation experiments were made to determine 
the effects of detaching small portions of the egg, in imitation 
of the observed spontaneous detachment of particles from the 
surface, to which reference has already been made. From 
repeated experiments it was conclusively shown that removal 
of small portions, and indeed of considerable portions, did not 
materially retard or modify the development of the eggs, or 
prevent their final development into perfect larvae. 
Again, the experiment of dividing eggs at the first cleavage, 
and at the second, and also of dividing them into several por- 
tions, was made in a considerable number of cases, with the 
results of obtaining from these fractions perfectly normal, 
though small, embryos, which continued to thrive and, finally, 
in the usual time transformed into perfectly nor- 
mal polyps. Pl. III, Fig. 11, shows one of these 
(a) e half-egg polyps which was isolated and reared 
e d under conditions which leave no shadow of doubt 
B (2 as to the genuineness of the case. And this is 
e £ only one of a considerable number, some from 
O "m » smaller portions, which: were similarly secured. 
Cos In Fig. 2 of text are shown a series of planulz of 
normal and artificially produced specimens, in 
which the relative sizes are clearly exhibited. 
On a preceding page attention was directed to a rather remark- 
able feature, namely, the natural origin of two embryos from a 
single egg, by a spontaneous division at some point during the 
segmentation. These facts would seem to show conclusively 
that so far as the hydroids are concerned there is no such pre- 
Fic. 2. 
