192 . COMPARATIVE ZOOLOGY. 
the yolk is large and colored, and consists of two parts— 
the formative, or germ-yolk, immediately surrounding the 
germinal vesicle; and the nutritive, or food-yolk, consti- 
tuting the greater part of the mass, by which the young 
animal in the egg-life is nourished. In the latter case, the 
young come forth more mature than where the food-yolk 
is wanting. 
As to form, eggs are oval or elliptical, as in Birds and 
Crocodiles; spherical, as in Turtles and Wasps; cylindric- 
al, as in Bees and Flies; or shaped like a hand- barrow, 
with tendrils on the corners, as in the Shark. The eggs 
of some very low forms are sculptured or covered with 
hairs or prickles. | 
The number of eggs varies greatly in different animals, 
as it is in proportion to the risks during development. 
Thus, the eggs of aquatic tribes, being unprotected by 
the parent, and being largely consumed by many animals, 
are multiplied to prevent extinction. The spawn of a 
single Cod contains millions of eggs; that of the Oyster, 
300,000. A Queen-bee, during the five years of her ex- 
istence, lays about a million eggs. 
Eggs are laid one by one, as by Birds; or in clusters, as 
by Frogs, Fishes, and most Invertebrates. The spawn of 
the Sea-snails consists of vast numbers of eggs adhering 
together in masses, or in sacs, forming long strings. 
As a rule, the higher the rank, the more care animals 
take of their eges and their young, and the higher the 
temperature needed for egg-development. In the majori- 
ty of cases, eggs are left to themselves. The fresh-water 
Mussel-shell (Unio) carries them between its gills, and the 
Lobster under its tail. The eggs of many Spiders are 
enveloped in a silken cocoon, which the mother guards 
with jealous care. Insects, as Flies and Moths, deposit 
their eggs where the larva, as soon as born, can procure 
its own food. Most Fishes allow their spawn, or roe, to 
