e 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. Since, however, men and women have descended from remote ancestors
who, in the manner of aquatic creatures, exercised functions of
sperm-extrusion and germ-extrusion that were exactly analogous in the two
sexes, without any specialized female uterine organization, the early
stages of human male and female foetal development still display the
comparatively undifferentiated sexual organization of those remote
ancestors, and during the first months of foetal life it is practically
impossible to tell by the inspection of the genital regions whether the
embryo would have developed into a man or into a woman. If we examine the
embryo at an early stage of development we see that the hind end is the
body stalk, this stalk in later stages becoming part of the umbilical
cord. The urogenital region, formed by the rapid extension of the hind
end beyond its original limit, which corresponds to what is later the
umbilicus, develops mainly by the gradual differentiation of structures
(the Wolffian and Muellerian bodies) which originally exist identically in
both sexes. This process of sexual differentiation is highly complex, so
that it cannot yet be said that t