Influence of Physical Agents on Development of the Frog. 239 
the 11th of March, and on the 31st of October the first was fully 
developed in the form of a frog; while other ova deposited on the 
same day, which were placed in a shady part of a room at 60° Fahr. 
and covered with several folds of black calico, were fully developed 
on May 22nd, twenty-three weeks earlier than those in the cellar. 
The experiment proved that the development of the frog depended 
upon the temperature, and not upon the presence of light. I found 
by other experiments that those in the light, and those deprived of 
light, were equally developed if placed at the same temperature. 
I observed that an excess of light retarded the development. 
1. On the Influence of Light on the Ovum. 
My formerexperiments not being parallel with those of Dr. Edwards, 
I was desirous of following his steps. Dr. Edwards says, in his work 
above referred to, Part iv. chapter 15, ‘On the Influence of Light 
upon the Development of the Body,’ ‘This process, previous to 
birth, is generally carried on in the dark ; there are, however, animals 
whose impregnated eggs are hatched, notwithstanding their exposure 
to the rays of the sun. Of this number are the Batrachians. I 
wished to determine what influence light independent of heat might 
exert upon this kind of development. ‘With this view I placed some 
spawn of the frog in water in a vessel which was rendered imper- 
meable to light by dark paper. The other vessel was transparent ; 
they were exposed to the same degree of temperature, but the trans- 
parent vessel received the rays of the sun. The eggs exposed to the 
light were developed in succession; of those in the dark, none did 
well ; in some, however, I remarked unequivocal indications of the 
transformation of the embryo.” 
Dr. Edwards does not mention the depth of water in the vessel in 
which he put the spawn of the frog, which he “‘ rendered impermeable 
to light.”’ If it were a few inches in depth, it would materially pre- 
vent the transformation of the embryo. 
I commenced my experiment in a pool which had been the habitat 
of frogs (the Rana temporaria) for several years. 
Experiment Ist.—I put a quantity of spawn, just deposited, into a 
box perforated with small holes, so as to admit a free current of water 
through it, and placed it about 3 feet below the surface of the water ; 
all the ova perished. 
The next experiments were made in an aquarium 20 inches deep, 
containing seventeen gallons of water at 60° Fahr. 
Experiment 2nd.—A quantity of spawn was put into the water, 
which fell to the bottom of the aquarium; the spawn when first 
deposited by the frog, is specifically heavier than the water*. The 
* It is found that the frog usually deposits her spawn in shallow water, often 
close to the edge of a pool favourable to its development; at first it is very ad- 
hesive, and adheres firmly to the ground or weeds where it is deposited. The 
same condition may be observed when the animal deposits her spawn in an earthen- 
ware vessel. In this neighbourhood the deposition begins about the end of 
February or the first days of March, and continues fourteen or sixteen days, when 
the frogs quit the water. 
hic 
