533 



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 1st. 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 

 ova enlarged as usual, but did not arrive satisfactorily through the 

 branchial state ; most of the ova appeared to undergo no change 

 whatever. 



* 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. 



