Remarks on the Physiology of the Egg. 371 
it is lined, constitute the purietes of the cavity, whose ex- 
tent in the recent egg scarcely exceeds in size the eye of a 
small bird: by incubation, however, it 1s extended to a 
considerable magnitude. That its most essential use is to 
oxygenate the blood of the chick, in my opinion there can 
be no donbt; but to establish completely the truth of such. 
a theory, it is necessary to discover the nature of the air by 
which it is inflated, and which has hitherto remained un- 
examined. We are informed by Buffon, that it is a pro- 
duct of the fermentation which the different parts of the 
eg undergo. If the count’s conjecture be established, it 
must be non-respirable, and therefore cannot discharge the 
office which such a theory would assign to it. To deter- 
‘mine this matter, and to discover also whether the process 
of incubation produces any change in its chemical consti- 
tution, I instituted the following experiments; viz. 
Experiment 1.—Twenty-one hen’s eggs newly laid, when 
punctured at their obtuse extremity, yielded only one cubi- 
cal inch of gas, which, when received in a jar, and subjected 
to the eudiometric test of Dr. Priestley, I found to be pure 
atmospherical air. 
Experiment i1.—Two eegs, after 20 days’ incubation, 
were opened under the surface of water, from which one 
cubical inch of gas was collected: this J also discovered to 
be atmospherical air, contaminated however with a small 
portion of carbonic acid, which I suspect to be derived 
from the venous blood of the chick, aud which seems to 
establish another most beautiful analogy between this mode 
of oxygenation, and réspiration after birth. 
From these results the following corollaries may be 
drawn! viz. 
1. The folliculus aéris before incubation contains atmo- 
spherical air. 
_ 2. No other chemical change takes place in the constitu- 
tion of the air, than a small inquination with carbonic acid. 
3. It gains by incubation an increase of volume, which 
takes place nearly in the ratio of ten to one, 
J must here remark, that its extent does not increase 
equally in equal successive portions of time, but observes a 
Tate of progression, which is accelerated as the Jatter stages 
of incubation advance: it seems, however, to arrive at its 
maximum of dilatationva few days previous to the exclusion 
of the animal. . 
In the eggs of inferior animals, the embryon does not 
appear to be oxygenated by any distinct apparatus, but, like 
the animal which it is hereafter to become, receives air 
Aag through 
