There is no reason to suppose that the germ-cell
and the sperm-cell are essentially different from each other. Sexual
conjugation thus remains a process which is radically the same as the
non-sexual mode of propagation which preceded it. The fusion of the nuclei
of the two cells was regarded by Van Beneden, who in 1875 first accurately
described it, as a process of conjugation comparable to that of the
protozoa and the protophyta. Boveri, who has further extended our
knowledge of the process, considers that the spermatozoon removes an
inhibitory influence preventing the commencement of development in the
ovum; the spermatozoon replaces a portion of the ovum which has already
undergone degeneration, so that the object of conjugation is chiefly to
effect the union of the properties of two cells in one, sexual
fertilization achieving a division of labor with reciprocal inhibition;
the two cells have renounced their original faculty of separate
development in order to attain a fusion of qualities and thus render
possible that production of new forms and qualities which has involved the
progress of the organized world.[74]
While in fishes this conjugation of the male and female elements is
usually ensured by the female casting her spawn into an artificial nest
outside the body, on to which the male sheds his milt, in all animals
(and, to some extent, birds, who occupy an intermediate position) there is
an organic nest, or incubation chamber as Bland Sutton terms it, the womb,
in the female body, wherein the fertilized egg may develop to a high
degree of maturity sheltered from those manifold risks of the external
world which make it necessary for the spawn of fishes to be so enormous in
amount.
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