THE HENS EGG. 



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innumerable villi ; it is here that the egg acquires a covering which 

 corresponds with the chorion of the Mammalian ovum, and becomes 

 partly calcified to form the shell. The inner part forms the shell-mem- 

 brane, and the outer part, becoming calcified, is the shell. These being 

 secreted and deposited outside the white, the egg is completed, and 

 then passes through the cloaca to be deposited. Birds are called ovip- 

 arous animals. 



The shell of the perfect egg (Fig. 119, e), is composed of 96 parts 

 of carbonate of lime, 2 of phosphate of lime, and 2 of animal matter. 

 The earthy matter is deposited in minute crystalline particles, em- 

 bedded in a delicate animal basis ; the shell is porous, admitting the 

 evaporation of fluid from within, and the passage of gaseous matters 

 in both directions. The shell-membrane , d, next within the shell, has 

 the appearance of tissue-paper, and consists of several layers of fine 

 matted fibres, running spirally, and composed, it is said, of solidified 



Fig. 119. 



Fig. 119. Section of the hen's egg. (Allen Thomson.) a, meroblastic yolk, inclosed by the vitelline 

 membrane, b, inner dense part of albumen. //, outer thinner or more fluid part, c, c, the chalazae. rf, 

 double shell membrane, e, the shell. /, the air-space between the two layers of the shell-membrane. 

 The section of the yolk shows the haloues,or concentric layers, also the central cavity or latebra, and the 

 canal leading up from this to the cicatricula or disc at its summit. 



albumen. At the larger end of the egg, the shell-membrane separates, 

 after a time, into two layers, between which, from the wasting of the 

 fluid of the egg, air finds its way from without through the shell ; the 

 interval is named the air-space, f ; it increases with the length of time 

 that the egg is kept ; it is not essential to, though it may assist in, 

 the respiration of the embryo-chick. The albumen, or white of the 

 egg, is more fluid next to the shell-membrane, but becomes denser in 

 its deeper parts, next to the yolk ; it consists of 11.5 per cent, of 

 albumen, 1 to 2 of fat, .5 of saline matters, chiefly chloride of sodium, 

 2 of extractives, and about 84 per cent, of water. Within the white, 

 the large yolk is held in its place, or moored, by the two coiled elastic 

 threads, named the chalazce, c, c, and, being lighter than the white, 

 floats in it. Moreover, owing to the chalazae being attached below 

 the centre, or horizontal axis, of the yolk, a particular portion of the 

 surface of the latter is always uppermost when the egg is laid upon 

 its side; during incubation, therefore, it is next to the hen's body, 



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