Another and perhaps more satisfactory mode of examining the yellow coat of the Graafian follicle in this stage, consists in slow maceration in a very weak preservative fluid (glycerine and water). The cells, which this coat contains in great abundance, can now be obtained separately for observation. They are seen to consist of a transparent cell-wall, filled with oil granules Cfig. 382. a). The average cells vary in diameter from -Eau" tO but many are smaller, and others lar ger. Occasionally a cell may be seen to have burst, its contents having escaped ; a few oil granules, however, may still be perceived ad hering to the cell-wall, the torn margins of which are very readily defined. There can be no doubt that these cells are the "peculiar granules" so frequently described and figured by Barry in his account of the various con ditions and stages of development of the ovisac.
The colour of the yellow coat—the so-called cmpus luteum —is not alike in all animals. In some of the Mammalia it is of a bright orange; in others it inclines to red. In Man, as already stated, the inner surface of the follicle, when rim is occasionally so loaded with bright red capillaries that the usual appearance is obscured, but its ordinary as pect presents the clear chrome yellow just described. That this yellow colour, like that of the yolk of the bird's egg, is due to the presence of the oil globules (fig. 382. b) which everywhere penetrate the tissues of this coat, is rendered sufficiently apparent : first, by the fact that treatment by ether, which dissolves out the oil granules, leaves the remaining membrane nearly white ; and secondly, that niaceration in water has, to a certain ex tent, the like effect, but in this case arising from the maceration, causing the animal membrane to swell and become opaque, thus obscuring its previous transparency, and ren dering the oily portions only faintly dis cernible through it, as judged by the naked eye, though they are still readily' discoverable under the microscope.
Third Stage. Period of Rupture or Dehis cence of the Follicle, and _Escape of the Ovum.
—This is tertned by Pouchet the period of partwition, in which, after the preparatory changes already described, the ovum quits the Graafian follicle in order to enter the Fallopian tube. It is therefore for the ovisac what the process of parturition is for the uterus, viz., the act by which the ovum, after being matured to a certain point of perfection, is expelled from its cavity.
The process by which the dehiscence of the follicle is effected in Manunalia is in some re spects different from that which causes the expulsion of the ovum, from its containing capsule, in the vertebrata below them. In birds, reptiles, and fishes, and, indeed, in the Invertebrata generally, the ovurn is of so large a size in comparison with the ovicap sule, that the simple increase of the former, as the time of the ovipont* approaches, is suffi cient to cause the bursting of the sac at the point where the coats have been prepared for rupture by previous attenuation. But in the Mammalia the bulk of the ovum bears so small a proportion to its containing follicle, that the ovum itself contributes in no degree to the rupture by which it is enabled to escape. In this process it remains a passive body, at least in a mechanical point of view, though doubtless it is the perfecting of the ovum which gives the vital impetus to that series of changes by which it is finally released from its first abode. But the act of parturition is accomplished by other means. The process by which this is effected has been compared by Blumenbach to the spon taneous bursting of an abscess. Here the process consists in an increasing accumulation of fluid within, conjoined with a gradual attenuation of some particular part of the containing walls. So many points of simi larity, indeed, may be traced between these two processes, that the term " inflammation " is employed by some authors in describing the preparatory changes in the Graafian fol licle.
The resistance which the ovum and other contents of the vesicle require to overcome before any portion of these can escape con sists, it must be remembered, in the combined opposition of no less than four membranes, in addition to any portion of the proper ovarian stroma which may intervene. These are, first, the ovisac ; then its capsule, united to the former, and with it constituting the Graafian follicle; thirdly, the tunica albuginea ; and fourthly, the peritoneal covering of the ovary. These four, shortly previous to the rupture, become so intimately united together that it is no longer possible to separate, nor is it easy always to distinguish them from each other, with the exception, however, of the innermost layer, which can generally be more easily traced than any of the rest, on account of its peculiar yellow colour.