62 ANIMAL PHYSIOLOGY. 
changes figured above, a general idea of their development may 
be obtained. 
It will be observed that throughout the series the nucleus of 
the cell is in every case preserved, and finally becomes the head 
of the male cell. Once more we are led to see the importance 
of this structure in the life of the cell. 
Fertilization of the Ovum.—The spermatozoén, lashing its way 
along, when it meets the ovum, enters it either through a special 
minute gateway (micropyle), or if this be not present—as it is 
not in the ova of all animals—it actually penetrates the mem- 
branes and substance of the female cell, and continues active 
till the female pronucleus is reached, when the head enters and 
the tail is absorbed or blends with the female cell. The nucleus 
of the male cell prior to union with the nucleus of the ovum 
undergoes changes similar to those that the nucleus of the 
ovum underwent, and thus becomes fitted for its special func- 
tions as a fertilizer; or perhaps it would be more correct to say 
that these altered masses of nuclear substance mutually fertil- 
ize each other, or initiate changes the one in the other which 
conjointly result in the subsequent stages of the development 
of the ovum. The altered male nucleus (male pronucleus), on 
reaching the female pronucleus, finds it somewhat ameboid, 
a condition which may be shared in some degree by the entire 
Fie. 66.—Fertilization of ovum of a mollusk (Elysia viridis). A. Ovum sending up a protu- 
berance to meet the spermatozoén. B. Approach of male pronucleus to meet the female 
pronucleus. #. PN, female pronucleus ; M. PN, male pronucleus ; S, spermatozodn. 
ovum. The resulting union gives rise to the new nucleus (seg- 
mentation nucleus), which is to control the future destinies of 
the cell; while the cell itself, the fertilized ovum (odsperm), 
enters upon new and marvelous changes. 
In reality this process was foreshadowed in the dim past of 
the history of living things by the conjugation of infusoria 
and kindred animal and vegetable forms. When lower forms 
(unicellular) conjugate they become somewhat amceboid sooner 
