306 Dr. Paris’s Remarks on 
and preventing evaporation from the surface of the animal, and 
the consequent change of temperature, may be the principal 
cause of this tenacity of life ? 
. It must however be remarked, that deviations of temperature 
are injurious and fatal in proportion only to the degree of vital 
energy which the ovular embryon possesses : hence germs of 
inferior vitality not only suffer the vicissitudes of heat and 
cold with impunity, but are developed by a less defined tem- 
perature. We therefore perceive, as we descend the scale of- 
oviparous beings, that those peculiar provisions which the eggs 
of perfect animals possess, for the regulation of their tempera- 
ture, cease to be essential, and therefore disappear. - 
The part of the egg to which I next beg to direct your atten- 
tion is the folliculus acris, or air-bag, placed at its obtuse extre- 
mity ; the nature of this follicle excited in me considerable in- 
terest, as I found that it had not been so fully investigated as its 
importance seemed to demand. ; 
The external shell, and the internal membrane by which it is 
lined, constitute the parietes of the cavity, whose extent in the 
recent egg scarcely exceeds in size the.eye of a small bird: by 
incubation, however, it is 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 doubt: but to establish com- 
pletely 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 unexamined, We are informed by Buffon, that it is a 
product of the fermentation which the different parts of the egg 
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 determine this matter, and 
to discover also whether the process of incubation produces any 
change 
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