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The germ cells reach their full development in special sex glands, the ova in the ovaries of the female and the spermatozoa in the testes of the male.
After the descendants of the primitive germ cells have increased, by ordinary
FIG. 9.-SCHEMA OF THE DEVELOPMENTAL HISTORY OF THE MATURE OVUM.
FIG. 10.-SCHEMA OF THE DEVELOPMENTAL HISTORY OF SPERMATOZOA.
cell division, to a number which is probably fixed and unchangeable, but which is not definitely known, they begin to increase in size, that is, they enter upon a period of growth, and at this time the female germ cells are called oocytes of the first order, oocytes I, and the male germ cells are called spermatocytes of the first order, spermatocytes I. Both the oocytes I and the spermatocytes I possess all the essential
parts of a typical animal cell, and, in addition, each has special peculiarities which differentiate it both from the germ-cells of the opposite sex and also from ordinary animal cells. Therefore the oocyte and the spermatocyte must be considered separately; but before this is done it must be noted that each oocyte I and each spermatocyte I is capable of producing only four descendants. The mitotic cell divisions by which the descendants are produced are called the maturation divisions, and they result, in the case of the oocyte, in the formation of one large functional cell-the mature ovum, and three small impotent cells-the polar bodies; whilst in the case of the spermatocyte the four descendants are of equal size and each becomes transformed into a presumably potent spermatozoon.
An ovum presents all the characteristic structural features of an animal cell, but it is peculiar on account of its relatively large size, the large size of its nucleus, and the possession of an investing membrane, the oolemma.
oocytes of the first order enter upon their
Whilst the growth of the oocyte and the thickening of the oolemma are still proceeding, a fluid-filled cavity appears in the stratum granulosum. Whether the cavity is due to the imbibition of fluid or to the dehiscence of the cells of the stratum granulosum is still uncertain, but,
As the young ova or
after its appearance, the cavity with its The corona radiata, which completely surrounds the
surrounding walls and the enclosed oocyte
is spoken of as a vesicular ovarian follicle.
ovum, is only represented in the lower part of the figure.
The cavity of the vesicular follicle gradu- 1. Corona radiata.
2. Granular layer.
5. Vitellus or Yolk.
ally increases, and, as it grows, it separates 3. Vitelline membrane. the oocyte and the cells of the stratum 4. Oolemma (zona pellucida). 8. Nuclear membrane. granulosum immediately around the oocyte from the remainder of the cells of the stratum, except in a small area where the two parts of the stratum granulosum still remain in direct continuity. When this condition is attained the cells of the stratum granulosum which immediately surround the oolemma are spoken of as the ovular cumulus; they enclose the oocyte, and, together with it, they form a bold promontory which projects into the cavity of the follicle.
When its full growth is attained each oocyte I is a comparatively large cell, which measures 200μ in diameter. It consists of a cell body which is surrounded by a definite enclosing membrane, the oolemma, and it contains (1) a nucleus, (2) a centrosome, (3) numerous granules called deutoplasmic or yolk granules, and (4) mitochondria.
The Oolemma.-The limiting membrane or oolemma is also called the zona pellucida, on account of its appearance under low magnifying powers, and the zona striata, because, under certain conditions, radial striae are seen in it when it is highly magnified. It is a strong, elastic membrane, which not only protects the oocyte from pressure, but probably also prevents the impregnated oocyte or zygote
from coming into close contact with the maternal tissues until it has attained the proper stage of development.
The exact origin of the oolemma is unknown. It must be formed either by the action of the cells of the stratum granulosum, or by the action of the oocyte, or by interaction between the two; but, up to the present, opinions regarding the origin are divided.
It is stated that processes of the cells of the ovular cumulus pass through the oolemma, forming the radial striae, and become continuous with or lie in close association with the protoplasm of the oocyte; and it is probable either that the processes are used as pabulum by the growing oocyte, or that they transmit nutritive material to the oocyte.
The Body of the Ovum.-The cell body, originally called the yolk, consists of spongio
FIG. 12. SCHEMA OF MATURATION OF OVUM, plasm and hyaloplasm.
EARLY PART OF PROPHASE OF FIRST DIVISION.
FIG. 13.-SCHEMA OF MATURATION OF OVUM IN
Each twin may be assumed to consist of a
The Deutoplasm.-The deutoplasm consists of a number of more or less highly. refractile granules, of varying size, which are embedded in the cytoplasm. They are largest in size and are most closely aggregated together in the region around the nucleus, where they form a definite deutoplasmic zone. But in the human ovum and in the ova of the majority of mammals, as contrasted with the ova of birds, reptiles, and amphibia, the amount of deutoplasm is relatively small; and for this reason the human ovum is classified as oligolecithal, the term telolecithal being applied to ova in which the deutoplasm is present in considerable amount, as in the ova of the frog; whilst the ova of birds, many reptiles, and the monotremes amongst mammals, in which the deutoplasm greatly preponderates over the cell protoplasm, are termed eutelolecithal.
The deutoplasmic granules are believed to serve as a store of nutritive material which is utilised during the early stage of the growth of the zygote, during which they disappear. (See note 2, p. 79.)
The Nucleus.The nucleus, formerly called the germinal vesicle, is a spherical vesicle of comparatively large size; its diameter, which measures 50μ, being onefourth of the diameter of the oocyte. It usually lies excentrically in the cytoplasm.
Its constituent parts are a nuclear membrane, surrounding the karyoplasm, which is separable, as in ordinary animal cells, into (1) an achromatic reticulum, the linin; (2) chromatic substance, which is embedded in or closely connected with the strands of the linin; and (3) the nuclear juice, which fills the meshes of the reticulum; and it contains usually one, but sometimes several nucleoli.
The Centrosome. The centrosome is not always very evident. It is usually present during the growth stage of the oocyte, and it disappears when the first maturation division commences. It may contain one or two centrioles, and it
lies in the region of the larger deutoplasmic granules by which its presence is frequently obscured.
The Mitochondria.-These minute particles can be demonstrated by suitable methods of fixation and staining.
The Maturation of the Ovum.-The process of maturation consists of two mitotic divisions, of which the first is heterotypical, and results in the reduction of the number of chromosomes, and the second is homotypical. The phenomena of the two divisions differ in some of their details from those of ordinary cell divisions, therefore a short account of them is necessary.
In the prophase of the first maturation division, the centrosome, the nucleolus, and the nucleus vanish, and an achromatic spindle appears at one pole of the oocyte, where it lies, at first, parallel with the surface; and the chromosomes are gathered around its equator. The number of the chromosomes is only half the typical number, and they are probably twin chromosomes (p. 11). There are no centrosomes at the poles of the spindle. After a short time the spindle rotates until it lies at right angles to its original position, and one pole, surrounded by a small amount of the cytoplasm, forms a projection, the first polar projection, on the surface of the oocyte (Fig. 14).
During the metaphase the twin chromosomes divide. In the anaphase the daughter chromosomes travel to the opposite poles of the spindle, and at the end of the anaphase one-half of the daughter chromosomes lies in the first polar projection and the other half in the body of the oocyte (Fig. 16).
In the telophase the first polar projection is separated from the body of the oocyte and Oocyte I ceases to exist, being converted into an oocyte of the second order, or oocyte II, and the first polar body, each of which contains half the typical number of chromo
somes at commence
separation of paternal and maternal parts)
FIG. 15.-SCHEMA OF MATURATION OF OVUM IN METAPHASE OF FIRST DIVISION. One pole of the spindle projects into the first polar bud, and the maternal and paternal parts of the chromosomes are separating from each other.
Polar bud with chro-
Chromosomes which remain in
FIG. 16.-SCHEMA OF MATURATION OF OVUM AT
END OF THE ANAPHASE OF THE FIRST DIVISION. Two chromosomes (paternal or maternal) lie in the first polar bud and two in the larger part of the ovum which becomes oocyte II. Second polar bud
First polar body.
The second maturation division occurs without the intervention of a resting stage, Chromosomes of e. without the reappearance of a nucleus in oocyte II.. A new achromatic spindle appears with the daughter chromosomes at its equator; it rotates, and one pole, surrounded by a small amount of cytoplasm, projects on the surface of the oocyte as the second polar projection (Fig. 17). In the metaphase the daughter chromosomes divide
homotypically into equal parts, and during FIG. 17.-SCHEMA OF MATURATION OF OVUM AT the anaphase the grand-daughter chromosomes move towards the poles of the spindle,
one-half entering the second polar projection and the other half remaining in the body of the oocyte. During the telophase the second polar projection is separated as the second polar body and the larger remaining part of the oocyte II becomes the mature ovum (Figs. 17 and 18).
Simultaneously with the division of the oocyte II into the second polar body and the mature ovum, the first polar body may divide into two parts. When that occurs four cells are present within the oolemma at the end of the maturation, i.e. the relatively large mature ovum and the three polar bodies (Fig. 19).
The details of the maturation of the human ovum are unknown, and the above account is based upon the phenomena which occur in other mammals. In mammals two polar bodies are invariably formed, but in many the first does not divide into two parts simultaneously with the formation of the second. The significance of the differences which occur is not at present fully understood. Each of the four descendants of the oocyte I contains half the typical number of
FIG. 18.-SCHEMA OF MATURATION OF OVUM AT
The chromosomes of oocyte II have separated
Second polar body
Parts of first polar
FIG. 19. SCHEMA OF MATURATION OF OVUM. END OF TELOPHASE OF SECOND DIVISION where the four descendants of oocyte I are the mature ovum, with half the original number of chromosomes, and three polar bodies.
chromosomes, and those in the mature ovum soon become enclosed in a new-formed nucleus, which is called the female pronucleus.
When the process of maturation is completed, the mature ovum differs from a typical animal cell, inasmuch as it probably possesses no centrosome and its nucleus contains the chromatic substance of only half the typical number of chromosomes.
The first maturation division always occurs whilst the oocyte is still in the ovary and before the spermatozoon has entered it. The second division takes place in the upper or middle part of the uterine tube, and always after the spermatozoon has entered the oocyte.
If the mature ovum does not meet with a spermatozoon it passes through the genital passages and is cast off and lost; or it breaks down, whilst still in the genital passages, into a detritus which disappears; but if it meets and unites with a spermatozoon a zygote is formed, from which a new individual may arise, and in that case the polar bodies persist until the zygote has undergone one or two divisions; but sooner or later they disappear, probably breaking down into fragments which are absorbed by the cells of the zygote.
Spermatocytes. When the male germ cells reach the period of growth they are called spermatocytes of the first order, or spermatocytes I, which correspond, morphologically, with oocytes I (Fig. 10).
The spermatocytes lie in the walls of. the tubules of the testes or male sex glands, where their descendants become converted into spermatozoa.
They differ from the oocytes I in three important respects: (1) they have no protective membrane corresponding with the oolemma of the oocyte; (2) they are not enclosed in follicles; (3) the spermatocytes are not surrounded by definite encircling layers of cells similar to the cells of the stratum granulosum.
As the spermatocytes lie in the walls of the tubules of the testes they are intermingled with other cells, the supporting and nurse cells, amidst which they undergo their maturation divisions, and their descendants become embedded in the nurse cells, where they are converted into spermatozoa. To a certain extent, therefore, the nurse cells may be looked upon as corresponding with the cells of the ovular cumulus which surround the growing oocyte.
After it has reached its full growth each spermatocyte I, like each oocyte I, can produce only four descendants, and the descendants, as in the case of the oocyte I,