KEPEODUCTION. 



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 {inembrana 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 chalazm; yet is al- 

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

 the albumen. The yolk is composed of oily granules 

 (about -^ of an incli in diameter), and is enclosed in a sac, 

 called the vitelline inemhrane, and disposed in concentric 

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

 part of the yolk whicii extends from the centre to a white 



Fia. 161. — Theoretic,! 1 Ejrg, 

 or Cell : u, vitelliue meiu- 

 braue ; y, oleagiuous pole ; 

 o, albuminous polo ; p, 

 Pnrkinjeau, or gei-miual, 

 vesicle ; w, Wagiieriau, or 

 germii)al, dot. 



Fi». 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: &, inner dense 

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

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

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



13 



