Picture - Cosmic Polymath

COMPOSITION OF THE HUMAN OVUM -i^jj
and altered ovum is in a condition to assume the cleavage necessary to the formation of the blastodermic membrane,
which in the higher vertebrates is the starting-point for the embryo and foetus. The changes which supervene during
the fertilising process bear a certain analogy to those occurring in the foetus itself (Plate xcii., Fig. 1, E, F, G, H, I,
page 396), so that the scheme of reproduction is, from the first, involved and complex, and the outcome of pre-
arrangement and design.
While the matrices of the two pronuclei appear to blend, their chromatin filaments maintain their identity.
The chromatin filaments form two sets of looped-v-shaped fibres of two each, and as these ultimately divide
longitudinally they make two sets of four each. At one stage they flatten and arrange themselves transversely
across the ovum, and so participate in, if they do not actually cause, the division of the ovum into two. They
subsequently open out transversely, and then at right angles, and in conjunction with radiating, concentric, ovoid,
and circular arrangements in the attraction-spheres complete the division of the ovum.
The several points discussed in connection with the maturation, discharge, and fertiUsation of the ovum are
illustrated at Plate xc, Plate xci., and Plate xcii.. Fig. 1, pp. 391, 393, and 396.
§ 72. Parts Entering into the Composition of the Human Ovum.
The human ovum in its entirety consists of the following parts : (1) globular zona pelluoida, otherwise called
zona radiata ; (2) globular vitelline membrane ; (3) globular vitellus or yolk, consisting of protoplasm and nutritive
material ; (4) globular germinal vesicle containing globular germinal spot.
The same parts are encountered in the rabbit.
The several parts of the ovum can only be made out by the aid of the microscope. In Plate Ixxxvi., page 383,
A, the ovum is seen under pressure and is more or less flattened. In B, C, the zona pellucida or radiata,
which has considerable resisting power, is purposely burst ; the protoplasm and nutritive material forming the
vitellus or yolk (6), and the germinal vesicle (c) having escaped. At D, the radiation in the zona pellucida (now
known as zona radiata) is indicated (a), as also the vitellus (6), germinal vesicle (c), and germinal spot {3,). The radiation
of the zona pellucida is beheved to contain pores, which permit the passage of noirrishment to the ovum and
provide entrances to the spermatozoa. The zona pellucida on leaving the Graafian foUicle is invested with a clear,
granular substance resembling the white of egg, which is supposed to contribute to the nourishment of the ovum, and
disappears when the ovum reaches the Fallopian tube. The protoplasm of the ovum on a casual examination is a
clear, apparently structureless, substance. On a closer examination, and when re-agents are employed, it is seen
to consist of a fine reticulum especially observable near the periphery of the ovum and around the germinal vesicle :
in other words, the protoplasm is differentiated. The other ingredients of the ovum (deutoplasm) contribute to
its nourishment.
The germinal spot, which measures -^^ to yj„ of an inch in diameter, consists of a nuclear membrane
containing a clear material, with strands of karyoplasm enclosing one or more nucleoli. If only one nucleolus occurs
it is known as the germinative spot {macula germinativa) . A certain amount of differentiation is also traceable
in the germinal vesicle. The old idea, that protoplasm is identical and structureless, may now be regarded as
exploded. The amount of nutritive material suppUed to the ovum is less in mammals than in fishes, reptiles, and
birds, and Professor F. M. Balfour conveniently divided ova into those with (a) accumulation of nutritive or
yolk matter at one pole (telolecithal ova—fish, reptile, and bird among vertebrates) ; (b) those with the accumulation
of yolk in the middle ovum {centrolecithal ova— ^in arthropods) ; and (c) those with yolk scattered equally and
in small amount through the protoplasm of the ovum {alecithal—mammals, amphioxus, echinoderms). This
distribution of the nutritive material (proto- and deuto-plasm) determines to a large extent the nature of the
segmentation in the several ova.
The development of the ovum after it leaves the ovary is at once intrinsic and extrinsic : intrinsic as regards
changes occurring in itself, and extrinsic as regards changes induced in it by the presence of the spermatozoon or
male element.
The intrinsic changes consist in the extrusion of certain constituents of the nucleus of the ovum known as the
" polar globules " : the extrinsic changes consisting in the intrusion into the nucleus of a spermatozoon or sperm-
cell to take the place of the extruded polar globules. The ovum or female element on its part makes room for the
spermatozoon or male element, and the latter takes full advantage of the arrangement. The male and female
elements come together, and either fuse and merge into each other's substance or, as B. v. Beneden thinks, they
maintain their identity, and mutually produce a double or compound cell, which is the starting-point of the new
being (Plate Ixxxvi., H to S inclusive, page 383).
The fertilisation of the ovum, it will be seen, is brought about by a union of the most intimate kind ;
the male
3 B