320 HELEN DEAN KING 
fore, to the number of tadpoles taken for rearing, and not to the 
number of eggs that had been used in making the experiment. 
The tadpoles lived chiefly on spirogyra, nitella, and various 
other water plants taken from ponds in which toads normally 
breed each year. Occasionally they were fed on finely ground 
fish or frogs’ muscle; but food of this kind that was not eaten 
within two or three hours was removed, as it fouled the water 
very quickly and so increased the mortality. Experience has 
shown that water plants, with their accompanying hordes of 
micro-organisms, form a food supply for toad tadpoles much 
superior to that used in any former experiments. (King, ’07 b, 
09 SAO): 
In Bufo the sexes cannot be distinguished until the tadpoles 
are approaching metamorphosis, and even when young toads 
live until their tails have been absorbed it is necessary to section 
the gonads in a considerable number of cases in order to ascertain 
the sex. Several attempts were made in past.years to feed young 
toads so that they might live until their gonads were well differ- 
entiated; but such attempts were failures, owing to the difficulty 
of obtaining a sufficient supply of small insects to feed a large 
number of individuals. Last spring it was found that young 
toads would eat various species of aphids and grow rapidly on a 
diet composed chiefly of these insects. As this food could be 
obtained in considerable abundance, nearly all of the individuals 
used in the various series of experiments were kept alive for about 
three weeks after they had completed their metamorphosis. By 
this time the sex glands were so well differentiated that the sex 
could readily be ascertained by examining the gonads in toto under 
a dissecting lens. 
The period of metamorphosis seems to be a critical one in the 
life history of toads reared under artificial conditions, as there is 
always an increased mortality at this time. All of the individu- 
als that died at this stage of development were preserved in 
Tellyesnicky’s fluid, which has been found far superior to corro- 
sive-acetic as a fixative for the gonads of Bufo, and their sex 
ascertained by means of sections. 
