150 THE PEOPLE'S PRACTICAL POULTRY BOOK. 



them from rolling about within the egg when it is moved. The white of 

 the egg being thus formed, a third membrane, or, rather, a double mem- 

 brane, much stronger than either of the first two, is formed around it, be- 

 coming attached to the chalazes of the second membrane, and tending still 

 more to keep all the parts in their relative positions. 



PROGRESS OF THE FORMATIONS. 



During the progress of these several formations, the egg gradually 

 advances about half way along the oviduct. It is still, however, destitute of 

 the shell, which begins to be formed by a process simliar to the formation 

 of the shell of a snail, as soon as the outer layer of the third membrane has 

 been completed. When the shell is fully formed, the egg continues to 

 advance along the oviduct, till the hen goes to her nest and lays it. 

 From ill-health, or accidents, eggs are sometimes excluded from the oviducts 

 before the shell has begun to be formed, and in this state they are called 

 wind eggs. 4 



THE EGG HAS SIX DIFFERENT ENVELOPES. 



Reckoning, then, from the shell inward, there are six different en- 

 velopes, of which one only could be detected before the descent of the egg 

 into the oviduct, the shell ; the external layer of the membrane lining 

 the shell ; the internal layer of same lining ; the white, composed of a 

 thinner liquid on the outside, and a thicker and more yellowish liquid on 

 the inside ; the bulgings, or chalaziferous membrane ; and the proper mem- 

 brane. One important part of the egg is 



THE AIR-BAG, 



placed at the larger end, between the shell and its lining membrane. This 

 is about the size of the eye of a small bird in new-laid eggs, but is in- 

 creased as much as ten or twelve times in the process of hatching. The 

 air-bag is of such great importance to the development of the chicken 

 probably by supplying it with a limited atmosphere of oxygen that, if the 

 blunt end of an egg be pierced with the point of the smallest needle, the 

 egg cannot be hatched. 



DOUBLE-YOLKED EGGS. 



Instead of one rudimental egg falling from the ovarium, two may be 

 detected, and will, of course, be inclosed in the same shell, when the egg 

 will be double-yolked. The eggs of a goose have, in some instances, been 

 known to contain even three yolks. If the double-yolked eggs be hatched, 

 they will rarely produce two separate chickens, but, more commonly, 

 monstrosities chickens with two heads, four legs, and the like. 



THE SHELL OF THE EGG. 



The shell of the egg, chemically speaking, consists chiefly of carbonate 

 of lime, similar to chalk, with a small quantity of phosphate of lime and 



