213 
of the Oviviviparous Shark, 
found that it was in all its chemical properties, the same with 
that formed in the oviduct of the frog, and when saturated 
with water, resembled the jelly in the oviduct of the dog- 
fish. 
The consideration of the properties of this jelly, with which 
the ova of the oviviviparous dog-fish and of the frog, are 
surrounded, and the purposes it is intended to answer, led me 
to take a view of the various modes by which the foetal 
blood is aerated in the different classes of animals. The pro- 
visions of nature for this very important purpose, form a 
beautiful series. 
The ova of many fish, as the salmon and trout, are laid in 
the sand and gravel, and the foetal blood of the embryo is 
aerated by means of the surrounding water ; these fishes, 
therefore, are found to spawn as near as possible to the 
sources of springs, where the water, coming from the earth, 
is in a state of high aeration. The ova of other fish, which 
spawn in waters less impregnated with air, are deposited on 
the leaves of water plants and weeds, that give out oxygen, 
by means of which, the embryo has its blood aerated. The 
tench and the pike are of these kinds. 
The ova of the perch are surrounded by a jelly as that in 
the frog, which imbibes water, probably for aerating the foetal 
blood. 
These observations were suggested to me, by my friend 
Professor Davy, who has made it a part of his amusements 
to attend to the habits of fishes. 
The ova of the oviparous shark, skate, and all that tribe, 
although too strong in their coats to be penetrated by the sea 
water, have natural apertures for its admission and escape, 
MDCCCX. F f 
