PRINCIPLES OF ANIMAL DEVELOPMENT. 203 



two membranes, was intended as a counterpoise for this loss ; for as 

 the chick grows, and of course the whole loses in weight, as also in 

 size, this air-bag swells, by a separation of the two membranes, and 

 fills up the space lost. So that this cavity may be said to be in size, 

 in proportion to the loss and condensation of parts which nourished the 

 chick ; and this is one of the purposes answered by it. 



As the whole volume of the chick and contents of the egg diminish 

 both in size and weight, it is necessary there should be a provision 

 for the first that the space might be filled : for this there is a provision 

 by means of the air-cell at the thick end, which, in the unincubated 

 egg, is extremely small, but increases as the contents of the egg 

 decrease ; and this increase of the air-cell is effected by a separation of 

 the two laminae of which the lining or internal membrane is composed. 



Principles governing the Formation of Animals. 



This production of animals out of themselves excites wonder, admira- 

 tion, and curiosity ; and tbis is commonly the case in effects whose 

 immediate causes are so obscure, more especially when we are ourselves 

 both effects and causes of the same. 



The first process set on foot in the formation of an animal is so small, 

 without that form which it afterwards gradually takes on, and its 

 situation so obscure, that its operation cannot be traced but by taking 

 it up at stated times, when we find a new part either added or come to 

 view, or a degree of perfection having taken place in the part. 



The larger the animal is in any one order, the more perfectly the 

 parts are seen as they rise to view, and, by this, the intermediate steps 

 in them are more within our view. 



If we were capable of following the progress of increase of the 

 number of the parts of the most perfect animal, as they first formed in 

 succession, from the very first to its state of full perfection, we should 

 probably be able to compare it with some one of the incomplete 

 animals themselves, of every order of animals in the Creation, being 

 at no stage different from some of the inferior orders. Or, in other 

 words, if we were to take a series of animals, from the more imperfect 

 to the perfect, we should probably find an imperfect animal, correspond- 

 ing with some stage of the most perfect 1 . But all our observations 



1 [The same philosophical idea seems to have governed Hunter in penning the 

 following passage : " We may also observe that the first rudiments of every animal 

 are extremely soft, and even the rudiments of the more perfect are similar to the 

 full-grown imperfect, and as they advance in growth they become firmer and firmer 

 in texture." — Croonian Lecture for the year 1782, Animal Economy, p. 268.] 



