ite some
remarkable discoveries of Professor Ganin, a Russian naturalist, on the
early stages of certain ichneumon parasites, which show some worm
features in their embryonic development. In a species of Platygaster
(Fig. 192, P. error of Fitch), which is a parasite on a two-winged gall
fly, the earliest stage observed after the egg is laid is that in which
the egg contains a single cell with a nucleus and nucleolus. Out of this
cell (Fig. 193 _A_, _a_) arise two other cells. The central cell (_a_)
gives origin to the embryo. The two outer ones multiply by subdivision
and form the embryonal membrane, or "amnion," which is a provisional
envelope and does not assist in building up the body of the germ. The
central single cell, however, multiplies by the subdivision of its
nucleus, thus building up the body of the germ. Figure 193 _B_, _g_,
shows the yolk or germ just forming out of the nuclei (_a_) and _b_, the
peripheral cells of the blastoderm skin, or "amnion." Figure 193 _C_
shows the yolk transformed into the embryo (_g_), with the outer layer
of blastodermic cells (_b_). The body of the germ is infolded, so that
the embryo appears bent on itself. Figure 193 _D_ shows the embryo much
farther advanced, with the two pairs of lobes (_md_, rudimentary
mandibles; _d_, rudimentary pad-like organs, seen in a more advanced
stage in _E_), and the bilobate tail (_st_). Figure 194 (_m_, mouth;
_at_, rudimentary antennae; _md_, mandibles; _d_, tongue-like appendages;
_st_, anal stylets; the subject of this figure is of a different species
from the insect previously figured, which, however, it closely
resembles) shows the first larva stage after leaving the egg. This
strange form, the author remarks, would scarcely be thought an insect,
were not its origin and farther development known, but rather a
parasitic Copepodous crustacean, whence he calls this the Cyclops-like
stage. In this condition it clings to the inside of its
host by means of its hook-like jaws (_md_), moving about like a Cestodes
embryo with its well known six hooks. The tail moves up and down, and is
of but little assistance in its efforts to change its place. Singularly
enough, the nervous, vascular, and respiratory systems (tracheae) are
wanting, and the alimentary canal is a blind sac, remaining in an
indifferent, or unorganized state. How long it remains in this state
could not be ascertained.

[Illustration: 193. Development of Platygaster.]

[Illustration