194 
COMPARATIVE ZOOLOGY. 
spot (< oicatricula ) on the outside cannot be hardened, even 
with the most prolonged boiling. The cicatricula, or em¬ 
bryo-spot—the part for which all the rest was made—is 
a thin disk of cellular structure, in which the new life 
first appears. This was originally a simple cell, but de¬ 
velopment has gone some way before the egg is laid. It 
is always on that side which naturally turns uppermost, 
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 conditions necessary for its development. There is 
another reason for this polarity of the egg: the lighter 
and most delicate part of the yolk is collected in its 
upper part, while the heavy, oily portion remains be¬ 
neath. 
In most eggs the shell and albumen are wanting. When 
the albumen is present, it is commonly covered by a mem¬ 
brane only. In Sharks, the envelope is horny; and in 
Crocodiles it is calcareous, as in Birds. 
The egg of the Sponge has no true vitelline membrane, 
and is not unlike an ordinary amoeboid cell. An egg is, 
t in fact, little more than a very large 
cell, of which the germinal vesicle is 
the nucleus. 
The size of an egg depends mainly 
upon the quantity of yolk it contains; 
and to this is proportioned the grade of 
development which the embryo attains 
w T hen it leaves the egg. 108 In the eggs 
Fig. tea.— Egg of sponge, of the Star-fishes, Worms, Insects, Mob 
n, nucleus. Magnified. ]uskg ( except the Cuttle - fishes), many 
Amphibians, and Mammals, the yolk is very minute and 
formative, i. e n it is converted into the parts of the future 
embryo. In the eggs of Lobsters, Crabs, Spiders, Cepha- 
lopods, Fishes, Reptiles, and Birds, the yolk is large and 
colored, and consists of two parts —the formative, or 
