REPRODUCTION. 191 
atricula, yolk, and vitelline membrane. The shell and 
albumen are often wanting. When the albumen is pres- 
ent, it is commonly covered by a membrane only, as in 
rns 
Fig. 159.—Egg of a Shark. 
Frogs; in Sharks, the envelope is horny; and in Croco- 
diles it is calcareous, as in Birds. 
The eggs of all animals, in their essentials, are alike in 
kind, but not in degree. The egg of the Ameeba is a 
mere cell, with a 7éght spot in one part of the contents. 
The egg of the Hydra differs in having this light spot 
sharply defined, and inclosed in a sac, or germinal vesi- 
cle. In the Rabbit’s egg, there is another sac, called ger- 
minal dot, inside the germinal vesicle; while the eggs of 
most of the higher animals show a nwcleolus within the 
germinal dot. An egg, therefore, closely resembles a cell, 
consisting of an outer and an inner sac and a dot, or a 
series of hollow concentric spheres. 
The size of an egg depends mainly upon the quantity 
of yolk it contains; and this is proportioned to the grade 
of development which the embryo attains when it leaves 
the egg." In the eggs of the Star-fishes, Worms, Insects, 
Mollusks (except the Cuttle-fishes), and Mammals, the yolk 
is very minute and formative, 7. ¢., it is converted into the 
parts of the future embryo. In the eggs of Lobsters, 
Crabs, Spiders, Cephalopods, Fishes, Reptiles, and Birds, 
