41?. The American Naturalist. [May, 
It is well known that the eggs of the higher animals, the 
mammals, are few in number, and that when fertilized, they 
are not discharged, but remain and develop in the body of the 
parent. Partly for this reason the embryology of the higher 
forms is much more difficult, but the eggs of the lower animals, 
Hke Crustacea, Corals and Starfishes, are deposited in very 
great numbers. The number of eggs laid by the edible Crab 
{Neptunus hastatus) of the Southern States, for instance, is 
estimated at 4>^ millions. The eggs are not only passed out 
of the body, but in many cases develop quite independently of 
the parent. Consequently a store of food called yolk, is laid 
up in the egg, as we see in the hen's egg, for the use of the 
growing embryo. 
We start with the fertilized germ cell the egg, although it 
should be remembered that there is a long series of events be- 
fore this is reached. The germinal cell itself is derived from 
other cells in the tissues of the mother and the tissues which 
compose the body, are themselves derived from the egg, and 
this cycle is repeated generation after generation. The male 
germinal cell, which in fertilization unites with the ovum, has a 
similar origin, so that the egg, from which the animal springs 
is not as simple a structure as one might suppose, but a micro- 
cosm in itself, containing as it must the hereditary germs of a 
As a rule an Qg^ does not develop unless it unites with an- 
other kind of cell, called the male germinal cell. This rule is 
however violated, in the case of the parthenogenetic insects, 
the Gall-wasps, Bees and Moths, and in some Crustacea, where 
the eggs develop without fertilization, and where the males are 
sometimes wanting. 
The egg of the shrimp, like that of the hen or tortoise 
consists of a large mass of food-yolk, surrounding the more 
essential part of the cell, — the nucleus, as it is called, the whole 
being enveloped by a protective membrane, the shell. Begin- 
ning then with the single egg-cell (which, if fertilized, is of 
course duplex in nature,) the animal is slowly developed by the 
division and differentiation of its products. The nucleus and 
