THE DEVELOPMENT OF THE FROG. 161 
added occasionally so as to keep the water as near one volume as 
possible. The tadpoles are commonly reputed to be vegetable 
feeders. I beg, therefore, to ask your careful attention to the 
following details of experiments which I have made to test this. 
I obtained four separate vessels for the tadpoles of similar size and 
shape, containing equal quantities of water with equal quantities of 
water plants. The tadpoles at sixteen days from the deposit of 
the egg, or two days from their emerging from the egg, I put ten 
selected specimens in each, if they are vegetable feeders ; you 
will observe that the conditions were all made equal. But for 
convenience, and that you may more readily follow this part of 
my subject I shall number them 1, 2, 3, and 4. I have just said 
that each vessel has an equal quantity of water, yet that water is 
very different so far as properties are concerned. No. r is supplied 
with filterings of pond water, so that with respect to animalcules it 
is of the richest description. No. 2 being supplied with half filter- 
ings and half pond water you will observe that in regard to 
animalcules this water can only be about half as rich as No. 1. 
No. 3 is supplied simply with pond water. No. 4 comes a step 
lower still, and is supplied with carefully filtered water. 
We have then No. 1 supplied with filterings. 
No. 2, half filterings and half pond water. 
No. 3, pond water. 
No. 4, filtered water. 
The results of those four methods of treatment I shall now lay 
before you in as few words as possible, and leave the inferences to 
be drawn from them in your hands. I may here add, however, 
that the four specimens stood together on a small table, so the 
conditions of light and heat were also equal. The following figures 
are the average of four years’ experiments for the complete develop- 
ment in each class. The denizens in these four vessels soon began 
to show a marked difference in their progress of development. 
The external gills were absorbed by No. 1 on the 2ist day; 
No. 2, 22nd day; No. 3, 23rd day; No. 4, 25th day. 
The outside gills are now replaced by internal gills developed 
on the gill-arches in the neck. While the tadpole continues in this 
stage of development its resemblance to a fish, with its inside gills, 
is very remarkable. The eyes of young tadpoles are, to all appear- 
ance, very rudimentary, and seem to be quite incapable of distinct 
vision, being covered with a blue glossy film. But No. 1 emerges 
out of this condition in about 30 days, while No. 4 does not get 
so far advanced as this until about the 4oth day. The others 
intermediate. The eyes of the tadpoles now present a bright 
sparkling appearance, and by this time, with a little management 
in the treatment of the tadpoles, their bodies may be in such a 
condition that it may be said, with truth, here is an object trained 
