human species has been unique, still it would not betoken any breach
of physiological continuity. Indeed, according to Weismann's not
improbable hypothesis touching the physiological meaning of polar
bodies, such a fact need betoken nothing more than a slight
disturbance of the complex machinery of ovulation, on account of
which the ovum failed to eliminate from its substance an almost
inconceivably minute portion of its nucleus.

Here, however, we must guard against an error which is frequently met
with in popular expositions of this subject. It is not true that the
embryonic phases in the development of a higher form always resemble so
many adult stages of lower forms. This may or may not be the case; but
what always is the case is, that the embryonic phases of the higher
form resemble the corresponding phases of the lower forms. Thus, for
example, it would be wrong to suppose that at any stage of his
development a man resembles a jelly-fish. What he does resemble at an
early stage of his development is the essential or groundplan of the
jelly-fish, which that animal presents in _its_ embryonic condition, or
before it begins to assume its more specialized characters fitting it
for its own particular sphere of life. The similarities, therefore,
which it is the function of comparative embryology to reveal are the
similarities of type or morphological plan: not similarities of specific
detail. Specific details may have been added to this, that, and the
other species for their own special requirements, after they had
severally branched off from the common ancestral stem; and so could not
be expected to recur in the life-history of an independent specific
branch. The comparison therefore must be a comparison of embryo with
embryo; not of embryos with adult forms.

* * * * *

In order to give a general idea of the results thus far yielded by a
study of comparative embryology in the present connexion, I will devote
the rest of this chapter to giving an outline sketch of the most
important and best established of these results.

Histologically the ovum, or egg-cell, is nearly identical in all
animals, whether vertebrate or invertebrate. Considered as a cell it is
of large size, but actually it is not more than 1/100, and may be less
than 1/200 of an inch in diameter. In man, as in most mammals, it is
about 1/120. It is a more or less spherical body, presenting a thin