i: 81 1 
III. Observatiojis on the changes the Ovum of the Frog undergoes 
during the formation of the Tadpole. By Sir Everard Home, 
Bart. V. P. R. S. 
Read November 25, 1824. 
Jn the year 1822, I laid before the Society a series of ob- 
servations on the progress of the formation of the chick in 
the egg of the pullet, illustrated by drawings from the pencil 
of Mr. Bauer, showing that in the ova of hot-blooded animals 
the first parts formed are the brain and spinal marrow. 
I have now brought forward a similar series on the pro- 
gress of organization in the ova of cold-blooded animals, 
illustrated in the same manner by microscopical drawings 
made by the same hand. 
By comparing together the first rudiments of organization 
in the ova of these very distinct classes of animals, I shall 
be able to prove that, in both, the same general principle is 
employed in the formation of the embryo. 
This enquiry has its interest considerably encreased, by the 
ova not being composed of similar parts. 
The ova of the frog, which have been selected for this in- 
vestigation, are found to have no yelk. If we examine these 
ova in the ovaria in which they are formed, we find them to ' 
consist of small vesicles of a dark colour ; when they enter 
the oviducts they enlarge in size, and acquire a gelatinous 
covering, which increases in quantity in their course along 
those tubes ; but the ova can neither be said to have acquired 
MDCCCXXV. M 
