306 Dr. Varis'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. AVe 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 tlierefore disappear. 



The part of the egg to which I next beg to direct your atten- 

 tion is the foUictilus aeris, 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 par/eto 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 

 jnature 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 difterent parts of the egg 

 undergo. If the Count's conjecture be established, it must be 

 Bon-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 



