REPRODUCTION. 



193 



egg. The structure of the hen's egg is more complicated. 

 The outside shell consists of earthy matter (lime) depos- 

 ited in a net-work of animal matter. 

 It is minutely porous, to allow the 

 passage of vapor and air to and fro. 

 Lining the shell is a double mem- 

 brane (membrana putaminis) resem- 

 bling delicate tissue-paper. At the 

 larger end, it separates to enclose a 

 bubble of air for the use of the chick. 

 Next comes the albumen, or " white," 

 in spirally arranged layers, within 

 which floats the yolk. The yolk is 

 prevented from moving towards either end of the egg by 

 two twisted cords of albumen, called chalazce ; yet is al- 

 lowed to rise towards one side, the yolk being lighter than 

 the albumen. The yolk is composed of oily granules 

 (about s^-q of an inch in diameter), enclosed in a sac, called 

 the vitelline membrane, and disposed in concentric layers, 

 like a set of vases placed one within the other. That part 

 of the yolk which extends from the centre to a white 



Fig. 161. — Theoretical Egg, 

 or Cell : v, vitelline mem- 

 brane ; y, oleaginous pole ; 

 a, albuminous pole ; p y 

 Purkinjean, or germinal, 

 vesicle ; w, Wagnerian, or 

 germinal, dot. 



Fig. 162. — Longitudinal Section of Hen's Egg before incubation: a, yolk, showing 

 concentric layers ; a\ its semi-fluid centre, consisting of a white granular sub- 

 stance—the whole yolk is enclosed in the vitelline membrane; 5, inner dense 

 part of the albumen ; outer, thinner part ; c, the chalazse, or albumen, twisted 

 by the revolutions of the yolk; d, double shell-membrane, split at the large end 

 to form the chamber,/; e, the shell ; h, the white spot, or cicatricula. 



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