6

556

granulosa,' those which surround the ovum itself form the 'proligerous disc,' ib. e, and the mass of cells thereto adhering is the 'cumulus.' The hyalinion,' or proper tunic of the ovum, thickens into the clear substance called zona pellucida,' f. The cells immediately around the ovum, as it ripens, elongate and become pyriform, with the pointed end attached to the zona': those of the cumulus diverge irregularly into the fluid intervening between them and the 'membrana granulosa' of the ovisac : but the four groups, defined by Barry' as retinacula,' ib. d, and fig. 559, g 2, may be an exceptional disposition.

a

b.

The ripe ovarian ovum, freed from its cellular precipitate, fig. 557, is inclosed in the thick transparent structureless hyalinion,' a: its vitelline contents are opaque through the abundance of granular yolk-substance, b: in this is the germinal vesi

cle,' with its nucleus or 'macula,' ib. c: it is more readily seen when the yolk is discharged from the ruptured ovum under pressure, as at b.

a

557

558

times a year or oftener: in the Human female it is menstrual. The number of ovisacs and ova which ripen at each rut varies according to the multiparity or uniparity of the species: in the Sow, e.g. fig. 558, there may be from four to six or more in each ovary; in the Ornithorhynchus, fig. 566, there are two only, and these limited to the left ovarium; in the Human female there is rarely more than one. The rut involves a determination of 1 As here shown it looks like a 'zone;' but is a bag, not a belt.

2 CIX.

blood to the ovarium, and especially to the swollen ovisac and its adventitious coverings: a thinning of these takes place at the most prominent part, to which the ovum tends: blood is extrava

559

sated into the ovisac, which, partly by absorption, partly by pressure, yields and gives issue to the ovum, fig. 559. This happens whether the male have access to the female in heat or not. In the Human kind the ovipont concurs with and probably occasions the menstrual discharge. The unimpregnated ovum may escape, as an impregnated one has sometimes done, into the abdominal cavity but, save that it probably perishes in its normal progress outward, it might be said that a woman lays an egg every time she menstruates an egg resembling in all essential structures that of the bird, but not exceeding th of an inch in diameter. Something like a sanguineous discharge has been observed in Quadrumana; but the more constant concomitant of the rut in that order is the swelling and vascularity of the external parts of generation. In the Mare an opaque white secretion is ejected per vulvam at the heat.

Ovum, with tunica granulosa, of the Rabbit, in the act of escaping from a ruptured Graafian follicle. CIX'.

§ 397. Corpus luteum.-After the escape of the ovum, with other contents of the ovisac, the walls of that cavity become thickened and altered in colour: in most Mammals they are partially everted at the ruptured orifice, fig. 566, b, b. In the Cow and Sheep such altered Graafian follicle' assumes a brickred colour; in the Sow a yellowish brown; and in the Woman the brighter colour led to its being called a corpus luteum.' In her the walls of the distended ovisac, compressed by the tunica albuginea and surrounding stroma, are thrown into delicate folds, fig. 560 the blood-clot which may have remained after the escape of the ovum is progressively absorbed. The plicated ovisac then contracts upon the cavity, and by the time the suc

2 In cccvIII. the diameter of the mature ovarian ovum is given, as being, in man, dog, cat, rabbit, rat, mouse pig cow to guinea-pig, of an inch.

ceeding ovisac with the ripening ovum has begun to protrude from the surface of the ovary, the old ovisac has lost its yellow colour, with much of its size, and has retired inward. This movement, with the collapse of the wall, depresses the cicatrix of the

6

aperture; and by these successive shrinkings and cicatrisations of the burst ovisacs, the ovary becomes marked by pits and furrows in advanced life. If the expelled ovum be not impregnated, the changes of the ovisac into the yellow convolute cavity, then into a small white stellate body, may occupy two months in the Human subject; but, if the maturation of successional ova be delayed by impregnation and its consequences, the first change goes on to a greater degree, and the corpus luteum' is not obliterated in less time than from thirteen to fourteen months: the inner coat, or original ovisac, is more thickened by a larger deposit of yellow oil-granules; it becomes more deeply plicated, is then compacted into a yellowish mass, and gains an adventitious white lining membrane, fig. 561. Rarely until after full gestation and delivery is the cavity obliterated:

it is then represented by a stellate linear figure surrounded by the corpus luteum,' which is ultimately absorbed.

561

§ 398. Impregnation.-After coitus the spermatozoa find their way to the Fallopian tubes, or oviducts, and might come into contact with the ovarian ovum, through the opening in

562

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the ovisac, prior to its expulsion, but they have never been traced so far. They were first seen, by MARTIN BARRY, t have penetrated the zona pellucida,' in a Rabbit's oviducal ovum, fig. 562. No definite single pore or 'micropyle' for the entry of the spermatozoon has been detected in that delicate evanescent tunic of the Mammalian ovum. The germinal vesicle,' or 'germ-cell,' disappears as such. A somewhat more opaque embryonal cell' succeeds, which may be, or includes, a combination of the nuclear matter of the sperm-cell with that of the germ-cell. Then follow the initial steps, figs. 563-565, which Barry's capital discovery showed to be the same essentially in Mammals as in all lower animals; and the entire yolk undergoes the cleavage-process in its combination with the progeny of the embryonal cell. Most of these initial steps are taken in the course of the impregnated ovum through the oviduct.

563

While in this narrow tube the ova are rolled to and fro by its peristaltic actions in a transparent fluid more or less abounding with spermatozoa; and the more of these get access to the yolk the more certain and complete is its segmentation. With the formation of the embryo-cell the yolk becomes separated by fluid from the 'zona pellucida,' and begins to rotate therein, as indicated by the arrows in fig. 562; one or two minute granular or oillike bodies may appear in the surrounding fluid.'

564

A division of the primary embryo-cell, with mutual repulsion of the two secondary ones, is followed by cleavage of the entire yolk, through attraction round each secondary cell, fig. 563, of the particles contiguous thereto. A repetition of this process issues in the four divisions of the germ-yolk, fig. 564; then in the eight, on until the whole is worked up into a

1 CCLXI", CCXLIX. for the same phenomena in Acephala (Unio and Anodon), p. 526; in Gastropods, p. 566.

mass of finely nucleate corpuscles; amongst which the qualities of the parent embryo-cell, due to impregnation, are thus equally distributed.

The eight-fold cleavage of the yolk has been observed three days after impregnation in the Rab

bit, four days in the Guineapig, and ten days in the Bitch: always in ova toward the uterine end of the Fallopian tube.

In the Bitch the smooth surface of the zona pellucida becomes irregularly flocculent, as if a granulo-mucous substance had been deposited thereon: in the Rabbit the ovum acquires a thick adventitious layer of albumen, fig. 565, a, before entering the uterus in the Guinea-pig the zona continues smooth; and,

565

after entering the uterus, on the fourth day, it grows fainter as the mulberry state of the yolk is there attained, and it disappears when the germ-mass is completed. The act of impregnation being thus consummated, ulterior changes with manifold modifications attend the development of the ovum in different Mammalia.

§ 399. Development of Monotremata.-The ripe ovarian ovum, though large in proportion to that in higher, especially placental, Mammals, is very much less than in Birds or Reptiles. Its external coat is thick, smooth, highly refracting-a true 'zona pellucida': the germinal vesicle is th of an inch in diameter: the larger proportion of vitelline matter, rich in granules and oil globules, is the chief distinctive character of the monotrematous ovum as a Mammalian one. I found two ovisacs with such mature ova in the left ovary of a female Ornithorhynchus, killed in September. In a specimen killed on the 6th of October (Yas River, New South Wales), the left ovary presented two discharged and altered ovisacs. The ova from these were situated at the upper part of the left uterus, and at the distance of about a line from each other. Each was spherical, and measured two lines and a half in diameter; the germ-mass, originally pale, had deepened to a yellow colour in the preserving liquor. The outer tunic had received no adventitious covering, but retained its smooth and polished exterior, and had not contracted any adherence to the uterine parietes. Each ovum was