308 -Dr V An I s's Remarks oti 



which it is hereafter to become, receives air through the medium 

 of spiracula, dispersed over the exterior involucrum. The de- 

 scription of the foUicuhis diris just delivered is taken from that 

 in tiie egg of our common hen. 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 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 foUicuhis a'ais 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 generally built on trees, and whose progeny are 

 born blind and forlorn. Thus the foUicuU in the eggs of fowls, 

 partridges, and moor-hens are of considerable extent, whilst 

 those in the eggs of crows, sparrows, and doves are extremely 

 contracted. The chick, therefore, of fowls and partridges has a 

 more perfect plumage, and a greater aptitude to locomotion, 

 than the callow nestlings of doves and sparrows. Such an 

 instance of the agency of oxygenation in the promotion and 

 increase of muscular power is not solitary in physiology; for the 

 history of ruminating animals will furnish us with a parallel 

 example. "Their cotyledons," observes the author of Zoonomici, 

 " seem to be designed for the purpose of expanding a greater 

 surface for the termination of the placental vessels, in order to 

 receive oxygenation from the uterine ones : thus the progeny 

 of this class of animals are more completely formed before their 

 nativity than that of the carnivorous classes. Calves therefore 

 and lambs can walk about in a few minutes after their birth ; 

 while kittens and puppies remain many days without opening 



their 



