THE UNIMPREGNATED OVUM.-The ovary is principally composed of Graafian vesicles, or follicles imbedded in areolar tissue, and abundantly supplied with blood-vessels. ( See cut and description, pages 49, 50. Each vesicle contains a single ovule; and consists of two membranes, the outer of which is in contact with the stroma of he ovary, while the inner tunic, or membrane granulosa, forms the immediate covering of the ovule and of he liquid in which it floats. For the ovule occupies but a very small part of the cavity of the Graafian vesicle, the remainder being filled with albuminous fluid, which, from its grater gravity supports the ovule upon it surface near the upper portion of the vesicle. Here the ovule comes in close relation with the inner surface of the membrane granulosa; and at this point there is found, collected upon the ovule, a zone of granules called the discus proliferus.
The ovule is composed externally of an envelope, the vitelline membrane, sometimes called the zona pellucida which after fecundation is known as the chorion l internally the ovule is composed in the vitellus, or yolk, a spherical, semi-solid mass, or granular organized liquid. the ovule is very small, measuring from the two hundred and fortieth to the one hundred and twentieth of an inch diameter.
Within the vitellus, and situated almost immediately beneath the vitelline membrane, is found a clear, colorless, transparent vesicle, of a rounded form, called the germinal vesicle. this may measure from the eight hundredth to the five-hundredth part of an inch in diameter.
Upon the surface of the germinal vesicle may be seen a dark spot, like a nucleus, called the germinal spot. While the rest of the contents of the vesicle is transparent, this is opaque; and its diameter may be stated to be not more than the two-hundredth or three-hundredth part of line. Plus Muller’s Elements of physiology, London, 1842, vol.ii., p.1471.
Such is a succinct description of the human ovum in its unimpregnated condition. But if we examine the same ovum some three weeks after it has become fecundated, we shall find it is wonderfully changed,.that were it not for the fact that all the various steps and stages of this remarkable transformation may be traced in the development of the ova of fishes, of birds and of the lower order of the mammalia, we might entirely fail of being able to recognize and prove the identity of the one with the other. ( The reader will fine a full, complete and very instructive exposition of the comparative anatomy of ovular development in the volume of Muller above referred to, pages 1507-1572.) The external tunic, or ovisac, becomes the chorion, whose cellular surface is extended into a number of villous prolongations, which form the channel through which the embryo is nourished by the fluids of the parent, until a more perfect communication is subsequently formed. The internal tunic, above described as the membrana granulosa becomes separate and distinct from the outer one, and is called the amnion. Between these two membranes now intervenes some considerable space, which is occupied by an albuminous liquid, in the midst of which is situated the umbilical vesicle. Within the amnion is found another ovule itself the germinal vesicle disappears, and a new cell, the embryo cell, arises in is place. and finally, the entire ovum. In the embryo cell, rises in its place. And finally, the entire ovum, with its external covering, is enveloped by a double covering, the deciduous membrane, which is developed and reflected from the inner surface of the uterus.
The manner of the escape of he unimpregnated ovule from the ovary has already been described, on page 247 of this work, to which reference is now to be made. The description of the manner in which impregnation is effected, will be found on page 296, which see the changes which occur in the uterus immediately after conception,, and the attendant condition of t ovary and Fallopian tubes, have been recounted on pages 299 and 300; these should be carefully reviewed. It remains now to describe the changes that occur in the ovum from the first possible moment of observation after impregnation, to the full development of the new being at the period just preceding parturition.
No change has ever et been observed immediately after fecundation until the escape of the ovule from the ovary into the abdominal extremity of the Fallopian tube. No ovule has yet been observed in he Fallopian tube still presenting either the germinal vesicle or the germinal spot. At that time, exactly, these feature of the ovule become transformed, has not yet transpired. But in it certain that the ovule, either while still remaining in the ovary, or in its way out from it, gradually loses both the germinal vesicle and the terminal spot. It may be possible that the spot disappears first, the vesicle subsequently; and it is certain that the space occupied by them becomes filled with granules. This single circumstance alone is sufficient to prove that conception must take place before the ovule enters the Fallopian tube; and that the ovules that have just caused the menstruation cannot be the same that are impregnated. The same circumstance also proves that there is one ovule for menstruation; and another for conception.
The only change observable in the ovum during its passage through the first half of the Fallopian tube, is the thickening of the vitelline membrane. During the passage of the ovum through the second portion of the tube, the vitellus evidently become more consolidated; and in concurrence with this change a thin, white fluid escapes, entirely surrounds the vitellus, the fills the interval between the vitelline membrane and the vitellus, occasioned by the condensation of the latter.
Other and very remarkable changes take place in the ovule as it descends through the second portion of the Fallopian tube. One of the more remarkable of these constitutes what is termed the segmentation of the vitellus, which, with its immediate consequences, may be thus described. The vitelline membrane continues to thicken, and the vitellus become divided into others, and so on, during he whole course of the descent into the uterus; each minute sphere dividing and subdividing until the ovum reaches the uterine cavity. The vitellus is thus completely dissipated, and what in the first instance was its exterior surface, come to resemble a mulberry seed in its appearance; while its cavity or interior is filled with a liquid containing an infinite number f minute granules. (This process of segmentation is illustrated in Figs. 35, 36 and 37, on the preceding page.)
These are termed vitelline spheres; they have a somewhat firmer consistency than original substance of the vitellus; and this consistency appears to increase as they successively multiply in number and diminish in size. At last they become so abundant as to be tended against the internal surface of the vitelline membrane. they have by this time become converted into true animal cells, which, adhering together by their adjacent edges, form a continuous organized membrane, called the blastodermic membrane. This constitutes the germ-mass, or plastic material, out of which the entire organization of he foetus is gradually evolved.
This blastodermic membrane is subsequently divisible into two distinct layers, which are known as the external and the internal layer. And as the blastodermic membrane, as a whole, represents all the foetal organization in its totality, so these two layers represent the commencement of all the particularly organs of he foetus; and the subsequent division all these organs into two distinct classes, those of the vegetative life and those of the animal life, respectively, finds its foundation in this pre-organic state of organization. For the internal layer of the blastodermic membrane produces the intestinal canal and all the organs of vegetative life; while the external layer is developed into the spinal column and the organs of animal life.
As nearly as can be determined, the time occupied by the human ovum in traversing he Fallopian tube is twelve days. And the movement is ways much more rapid in the first than in the second half of its descent; when the nearer it approaches the uterus the slower it moves probably on account of the increasing narrowness of the tube. The ovum is known to enlarge decidedly during its continuance in the tube; during which time the the product of conception must be nourished by the granules contained in its interior fluid. When the ovum approaches the uterine cavity, we find in it simply the vitelline membrane much thatched and surrounded by a dense layer of albumen which it has collected in passing through the tube, layer of albumen which it has collected in passing through the tube, and the vitellus containing some graduations remaining from the decomposition, of he mulberry like body. The vitelline granulations as they disappear, give place to a perfectly white, transparent liquid, In their disappearance, these granulations seem to the condensed, the by adhering to one of the, they form of a new vesicle which lines the first. This fact may be easily demonstrated by macerating the ovule in water, when the new vesicle will be thrown of by the water percolating through the vitelline membrane and be seen to lay corrugated in distinct folds, in the above- mentioned transparent liquid. This new membrane, or vesicle, is called the blastodermic vesicle. Whilst this process is going on, the albumen collects as above stated, be come absorbed, and the vitelline membrane is much thinned. the ovule, now for the first time begins to be fixed, and can no longer be blown about, as before, on the surface of the mucous membrane.
