packed
together. Between these ovarian cells the egg originates, and is at
first a mere granule, so minute, that, when placed under a very high
magnifying power, it is but just visible. This is the incipient egg,
and at this stage it differs from the surrounding cells only in being
somewhat darker, like a drop of oil, and opaque, instead of transparent
and clear like the surrounding cells. Under the microscope it is found
to be composed of two substances only: namely, oil and albumen. It
increases gradually, and when it has reached a size at which it requires
to be magnified one thousand times in order to be distinctly visible,
the outside assumes the aspect of a membrane thicker than the interior
and forming a coating around it. This is owing not to an addition from
outside, but to a change in the consistency of the substance at the
surface, which becomes more closely united, more compact, than the
loose mass in the centre. Presently we perceive a bright, luminous,
transparent spot on the upper side of the egg, near the wall or outer
membrane. This is produced by a concentration of the albumen, which
now separates from the oil and collects at the upper side of the egg,
forming this light spot, called by naturalists the Purkinjean vesicle,
after its discoverer, Purkinje. When this albuminous spot becomes
somewhat larger, there arises a little dot in the centre,--the germinal
dot, as it is called. And now we have a perfect cell-structure,
differing from an ordinary cell only in having the inner sac, inclosing
the dot, on the side, instead of in the centre. The outer membrane
corresponds to the Ectoblast, or outer cell sac, the Purkinjean vesicle
to the Mesoblast, or inner cell sac, while the dot in the centre answers
to the Entoblast. When the Purkinjean vesicle has completed its growth,
it bursts and disappears; but the mass contained in it remains in the
same region, and retains the same character, though no longer inclosed
as before.

At a later stage of the investigation, we see why the Purkinjean
vesicle, or inner sac of the egg, is placed on the side, instead of
being at the centre, as in the cell. It arises on that side along which
the axis of the little Turtle is to lie,--the opposite side being that
corresponding to the lower part of the body. Thus the lighter, more
delicate part of the substance of the egg is collected where the upper
cavity of the animal, inclosing the nervous system and brain, is to
be, whil