the fowl and duck would be indistinguishable in the
early stages of development; and that in two species so closely similar
that I was long in doubt whether they were distinct species, viz.,
_Peripatus capensis_ and _Balfouri_, it would be useless to look for
embryonic differences; yet I can distinguish a fowl and a duck embryo on
the second day by the inspection of a single transverse section through
the trunk, and it was the embryonic differences between the Peripatuses
which led me to establish without hesitation the two separate
species.... I need only say ... that a species is distinct and
distinguishable from its allies from the very earliest stages all
through the development, although these embryonic differences do not
necessarily implicate the same organs as do the adult differences" (p.
39).

Hertwig interprets this fact of the specific distinctness of closely
allied embryos in the light of the preformistic conception of heredity.
According to this view the whole adult organisation is represented in
the structure of the germ-plasm contained in the fertilised ovum, from
which it follows that the ova of two different species, and also their
embryos at every stage of development, must be as distinct from one
another as are the adults themselves, even though the differences may
not be so obvious. If this be the case there can be no real
recapitulation in ontogeny of the phylogeny of the race, for the
egg-cell represents not the first term in phylogeny, but the last. The
egg-cell _is_ the organism in an undeveloped state; it has a vastly more
complicated structure than was possessed by the primordial cell from
which its race has sprung, and it can in no way be considered the
equivalent of this ancestral cell.

Hertwig puts this vividly when he says that "the hen's egg is no more
the equivalent of the first link in the phylogenetic chain than is the
hen itself" (p. 160, 1906, b).

If ontogeny is not a recapitulation of phylogeny, how is it that the
early embryonic stages are so alike, even in animals of widely different
organisation? Hertwig's answer to this is very interesting. He takes the
view that many of the processes characterising early embryonic
development are the means necessarily adopted for attaining certain
ends. Such are the processes of segmentation, the formation of a
blastula, of cell-layers, of medullary folds where the nervous system is
a closed tube, the formation of the notochord as a necessa