transparent envelope, called the _zona pellucida_, which
contains--first, the protoplasmic cell-substance or "yolk," within which
lies, second, the nucleus or germinal vesicle, within which again lies,
third, the nucleolus or germinal spot. This description is true of the
egg-cells of all animals, if we add that in the case of the lowest
animals--such as sponges, &c.--there is no enveloping membrane: the
egg-cell is here a naked cell, and its constituent protoplasm, being
thus unconfined, is free to perform protoplasmic movements, which it
does after the manner, and with all the activity, of an amoeba. But
even with respect to this matter of an enveloping membrane, there is no
essential difference between an ovum of the lowest and an ovum of the
highest animals. For in their early stages of development within the
ovary the ova of the highest animals are likewise in the condition of
naked cells, exhibiting amoebiform movements; the enveloping membrane
of an ovum being the product of a later development. Moreover this
membrane, when present, is usually provided with one or more minute
apertures, through which the spermatozooen passes when fertilizing the
ovum. It is remarkable that the spermatozoa know, so to speak, of the
existence of these gate-ways,--their snake-like movements being directed
towards them, presumably by a stimulus due to some emanation
therefrom[12]. In the mammalian ovum, however, these apertures are
exceedingly minute, and distributed all round the circumference of the
pellucid envelope, as represented in this illustration (Fig. 32).

[12] The spermatozooids of certain plants can be strongly attracted
towards a pipette which is filled with malic acid--crowding around
and into it with avidity.

[Illustration: FIG. 30.--Ovarian ovum of a Mammal, (_a_) magnified
and viewed under pressure, (_b_) burst by increased pressure, with
yolk and nucleus escaping: (_c_) the nucleus more freed from
yolk-substance. (From _Quain's Anatomy_, after Allen Thomson.)]

[Illustration: FIG. 31.--Amoeboid movements of young egg-cells, _a_,
Amoeboid ovum of _Hydra_ (from Balfour, after Kleitnenberg); _b_,
early ovum of _Toxopneustes variegatus_, with pseudopodia-like
processes (from Balfour, after Selenka); _c_, ovum of _Toxopneustes
lividus_, more nearly ripe (from Balfour, Hertwig). A1 to A4, the
primitive egg-cell of a Chalk-Sponge (_Leuculmis echinus_), in four
s