STRUCTURE OF EGGS. 115 



placed at the obtuse extremity, and well described 

 by Dr. Paris. " The external shell," he says, " and 

 the internal membrane by which it is lined, con- 

 stitute the parides [walls] 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 

 completely the truth of such a theory, it is necessary 

 to discover the nature of the air by which it is in- 

 flated, and which has hitherto remained unexamined." 

 From experiments made to ascertain this point Dr. 

 Paris concludes that " before incubation it contains 

 atmospherical air. No other chemical change takes 

 place in the constitution of the air, than a small 

 inquination with carbonic acid. It gains by incuba- 

 tion an increase of volume, which takes place nearly 

 in the ratio of 10 to 1. I must here remark, that 

 its extent does not increase equally in equal succes- 

 sive portions of time, but observes a rate of progres- 

 sion, which is accelerated as the latter stages of 

 incubation advance : it seems, however, to arrive at 

 its maximum of dilatation a few days previous to 

 exclusion of the animal. 



" The same apparatus exists in the eggs of all 

 birds, and contains a similar air : its capacity, how- 

 ever, does not seem to vary either with the size of 

 the egg or of the bird to which it belongs ; but I 

 think I have discovered a beautiful law by which its 

 extent is modified. I have uniformly found, as far 

 as my contracted inquiries have led me, that the 

 folliculus aeris is of greater magnitude in the eggs of 

 those birds which place their nests on the ground, 

 and whose young are hatched, fledged, and capable 

 of exerting their muscles as soon as they burst from 

 their shell, than in the eggs of those whose nests are 



