194 
COMPARATIVE ZOOLOGY. 
spot ( cicatricula ) 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, 
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 
when it leaves the egg . 108 In the eggs 
of the Star-fishes, Worms, Insects, Mol- 
lusks (except the Cuttle-fishes), many 
Amphibians, and Mammals, the yolk is very minute and 
formative, i. e ., it is converted into the parts of the future 
embryo. In the eggs of Lobsters, Crabs, Spiders, Ceplia- 
lopods, Fishes, Reptiles, and Birds, the yolk is large and 
colored, and consists of two parts — the formative, or 
Fig. 163. —Egg of Sponge 
n , nucleus. Magnified. 
