THE MAMMALIAN OVUM. 947 



by a small opening, and the ovum, with some of the nucleated cells of 

 the membrana granulosa, a few of which have now acquired a club- 

 shape and cling to the ovum in stellate masses, enters the funnel- 

 shaped end of the so-called Fallopian tube, which corresponds with the 

 oviduct in the Bird. Down this tube the ovum descends by peristaltic 

 action, perhaps aided by the movements of cilia, into the cavity of the 

 uterus or womb, within which it undergoes its future development. 

 Each ovary has its own Fallopian tube. 



The emptied Graafian follicle, the walls of which have previously 

 become thickened and vascular, is first filled with effused blood, which 

 becomes absorbed. A yellow substance is then deposited in its coats, 

 and, becoming plicated, forms the so-called corpus luteum; this is vas- 

 cular, and consists of cells arranged in a columnar manner, mixed with 

 soft fibres and a yellowish fat. It gradually disappears. 



In the meantime, the lining membrane of the uterus, with its colum- 

 nar ciliated epithelium, has become thickened and more vascular, and 

 certain glands within it are highly developed, so as to form perpendic- 

 ular tubuli. The intertubular substance also undergoes hypertrophy, 

 soon containing many new cells, with much fluid and fatty matter, 

 thus forming a soft nutrient matrix, from which, by mere imbibition, 

 the early ovum may be nourished. The superficial stratum of the 

 altered mucous membrane, thus modified, becomes changed into the 

 soft, pulpy, opaque membrane, known as the decidua, because it is 

 thrown oft' with the embryo at birth. This membrane consists, after a 

 time, of two layers, known as the decidua vera, and the decidua reflexa. 

 The decidua vera is cribriform, being perforated by little orifices cor- 

 responding with the enlarged uterine glands; it lines the uterus, but 

 is wanting at the orifice, and also at the openings of the two Fallopian 

 tubes. It contains tortuous arteries proceeding from the uterus, to- 

 gether with large veins and venous sinuses, fine areolar tissue, nucleated 

 cells, and soft granular matter. As this structure grows, it ultimately 

 forms the maternal portion of the placenta, with its arteries, and ve- 

 nous sinuses or lacuna?, which is intended to convey nourishment to the 

 future embryo, and to accomplish the respiratory changes in its blood. 

 To some part of the decidua vera the ovum soon becomes adherent, 

 whilst the decidua reflexa, as its name implies, covers in the ovum, 

 either owing to a sinking-in of that little body, or to a rising-up of the 

 decidual membrane. Ultimately the two portions of the decidua coa- 

 lesce, or the reflected part disappears. At the same time that the 

 decidua generally is becoming converted into the maternal portion of 

 the placenta, the ovum itself, having been fertilized, grows rapidly, 

 and undergoes remarkable changes in its interior some relating to 

 the formation of the embryo itself, others referring to the coats or 

 membranes which constitute its means of protection and of attachment 

 to the maternal placenta. The outer vitelline membrane, which, from 

 its thick and transparent albuminoid character, is named the zona pel- 

 lucida, having lost the club-shaped adherent cells of the membrana 

 granulosa, has developed around it a thin, but strong whitish mem- 

 brane, named the chorion, corresponding with the shell-membrane of 

 the Bird's egg. The chorion is a fibrous membrane, having a layer of 



