190 COMPARATIVE ZOOLOGY. 
inch in diameter), inclosed 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 spot (cccatricu- 
da) on the outside can not be hardened, even with the 
most prolonged boiling. The cicatricula, or embryo-spot 
—the part for which all the rest was made—is a thin disk 
of cellular structure, in which the new life first appears. 
It is always on that side which naturally turns uppermost, 
Fig. 158.—Longitudinal section of Hen’s Egg before incubation: a, yolk, showing 
concentric layers; @’, its semi-fluid centre, consisting of a white granular sub- 
stance—the whole yolk is inclosed in the vitelline membrane; b, inner dense 
part of the albumen ; 0’, outer, thinner part; ¢, the chalaze, or albumen, twisted 
by the revolutions of the yolk; d, double shell-membrane, split at the large end 
to form the chamber f; e, the shell; h, the white spot, or cicatricula, and under 
it the germinal vesicle of Purkinje, or nucleus, which is afterward ruptured, and 
becomes invisible. 
for the yolk can turn upon its axis; it is, therefore, al- 
ways nearest to the external air and to the Hen’s body— 
two necessary conditions for its development. There is 
another reason for this polarity of the egg: the lighter 
and most delicate part of the yolk, the cicatricula, is col- 
lected where the upper cavity of the animal, inclosing the 
nervous system, is to be; while the heavy oily portion re- 
mains beneath, where the lower cavity, inclosing the or- 
gans of nutrition, is afterward developed. 
The essential parts of any egg are the germ-cell, or cic- 
