6

of observations by which the germinal vesicle,' the germina spot,' the yolk, and yolk-membrane, were determined in the minute opaque sphere; and thus was the ovarian egg of the Mammal finally made known.

The ovisac and ovum appear later in the ovary than do the seminiferous tube and spermatoon in the testis. The first-formed

555

elements in the fetal ovary are those called cells' and 'cellnuclei' next appear roundish groups of such primary cells, rather more opaque than the rest of the previously uniform mass, fig. 555, A. A film soon condenses round these purposive groups, ib. B, upon the inner surface of which forms an epithelial precipitate from the fluid and granules of the interspaces of the contained primary cells: within the 'ovisac' thus formed a larger nucleate cell becomes visible, which is the beginning of the ovum. As the ovisac expands the proportion of fluid to the formified particles increases, and the latter are attracted to the contiguous surfaces, some to that of the ovisac, which thus becomes lined by a thicker layer of cells, others to the ovum, accumulating around it. With the enlargement of the ovisac, the

stroma

ovarii' condenses around its delicate membrane, fig. 556, b, to form the theca folliculi' of Baer. This vascular covering of the ovisac, ib a, with the proper wall, ib. b, constitutes the Graafian vesicle or follicle.' The stratum of nucleate cells lining the ovisac is termed membrana

6

Voce ovuli semper usus sum, quia vesicula Graafiana ipsa ovum refert, respecto ovario, ex ovulo autem, fit ovum fetale.' . . . Ex quo concludo: quo diutius in corpore materno fetus foventur, eo magis jam primitus exculta videtur ovi vesicula innata, quæ in mammalibus eo pervenit ut omnis ovi virtutes in sese recipiat et relique ori partes parvi momenti extranea quasi fiant.' CCXLIX". p. 33. [The italics are V. Baer's.] CCL". CCLI". CCLII".

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 Barry2 as retinacula,' ib. d, and fig. 559, g 2, may be an exceptional disposition.

Graaflan vesicle and ovum, Rabbit; magn. CIX.

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.

§ 396. Ovipont.-The maturation of ova occasions the

rut' or 'heat' in many brutes it is annual; in the Ferret twice a year; in the domestic Rabbit, Cat, Hog, Bitch, it may recur three

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 Ovary, Sow, with ripe follicles burst;

2 CIX.

As here shown it looks like a 'zone;' but is a bag, not a belt.

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

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

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

1

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.

§ 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, 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

Corpus luteum,' after escape of ovum, Human. CCXLVI".

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

the ovisac, prior to its expulsion, but they have never been
traced so far. They were first seen, by MARTIN BARRY, to
have penetrated the zona pellucida,' in a Rabbit's oviducal ovum,
No definite single pore or
fig. 562.
'micropyle' for the entry of the sperma-
tozoon 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 under-
goes the cleavage-process in its combina-
tion 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

6

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 parti-. cles contiguous thereto. A repetition of this process issues in the four divisions of the germ-yolk, fig. 564; then in the eight, as in fig. 565; and so 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.

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