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 aniinal 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 jputaminis ) 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 -2-37J- of an inch in diameter), and is 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 
Fia. 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; b, inner dense 
part of the albumen ; outer, thinner part; c, the chalazae, 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. 
13 
a 
Fio. 161. — Theoretical Egg, 
or Cell: r, vitelline mem¬ 
brane ; y , oleaginous pole; 
a , albuminous pole; p, 
Purkinjean, or germinal, 
vesicle; w, Wagnerian, or 
germinal, dot. 
