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 chalazcB; 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 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 
Fio. 161. — Theoretical Egg, 
or Cell: r, vitelline mem¬ 
brane ; y, oleaginous pole; 
a, albuminous pole; p , 
Purkiujean, 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; b, 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 t the shell; h , the white spot, or cicatricula. 
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