1821 .] Dr. Pan's on the Physiology of the Egg. 3 



takes place nearly in the ratio of 10 to 1 ; it must, howeyer, be 

 remarked that its extent does not increase equally in equal suc- 

 cessive portions of time ; but that it observes a rate of progres- 

 sion which is accelerated as the stages of incubation advance, 

 although it seems to arrive at its maximum of dilatation a few 

 days previous to the exclusion of the animal. 



Some naturalists have conjectured that the use of this appa- 

 ratus is to furnish the air with which the feathers are inflated : 

 this idea hardly requires a serious refutation ; we detect the 

 same receptacle in the eggs of those birds that are hatched 

 unfledged. Its essential purpose is undoubtedly to oxygenate 

 the blood of the embryon, and we accordingly find that what- 

 ever obstructs the inflation of this follicle, and the renewal of its 

 air, destroys the life of the chick. The experiments of Reaumur 

 offer abundant proofs of this truth. In his attempts to develope 

 the egg by the heat of dung, they, for a long time failed, o wing- 

 to a circumstance which he afterwards discovered to depend upon 

 the impurity of the atmosphere. He also varnished eggs so as 

 to prevent the access of air ; and he found that when placed 

 under the hen, they invariably perished. Spallanzani instituted 

 many experiments with the same view. " I have often," says 

 he, " placed the eggs of terrestrial and aquatic insects under 

 the receiver of an air-pump, but none ever hatched in this situa- 

 tion, although in every other respect in a condition to have 

 done so;" and Boerhaave offers his testimony upon the same 

 subject in the following words : " Ovula quorumcunque insecto- 

 rum, in vitris accurate clausis, non producunt." We see the 

 importance, therefore, of that provision by which the egg is 

 occasionally ventilated by the migration of the parent ; it is a 

 fact well known in the farm yard, that turkeys frequently destroy 

 or smother their eggs by a too constant and assiduous attention. 

 The air follicle may also have a secondary office to perform, 

 to assist in producing necessary changes in the albumen and 

 vitellus by the chemical action of its air. Such then is the 

 nature of this organ in the egg of the common hen, from which 

 this description is taken ; the same apparatus exists in the eggs 

 of all birds, and contains a similar air ; its capacity, however, 

 does not seem to vary either with the size of the egg, or of the 

 bird to which it belongs ; but I think that I have "discovered a 

 beautiful law by which its extent is regulated. I have uniformly 

 found (as far as my contracted inquiry has led me), that the fol- 

 liculus aeris is of greater magnitude in the eggs of those birds 

 who 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 the foUkuli of those whose 

 nests are generally built on trees, and whose progeny are born 

 blind and forlorn. Thus the folliculi of the eggs of hens, par- 

 tridges, and moor-hens, are of considerable extent ; while those 

 of the eggs of crows, sparrows, and doves, are extremely con- 



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