ACTION OF THE FEMALE. 683 



Mammalia, a large amount of granular matter is present ; and this arranges 

 itself into some peculiar structures discovered by Dr. Barry, and presently to 

 be described. The membrane which surrounds the yolk in Mammalia has 

 received, on account of its thickness and peculiar transparency, the designation 

 of zona pellucida. The several parts of the Ovum now described are shown 

 in Fig. 5, Plate I. 



906. From the researches of Dr. Barry on the early development of the 

 Ovum, it appears that the Germinal Vesicle is the part which can first be 

 distinctly traced. In Fig. 1 (Plate I.) is seen a representation of one of its 

 incipient stages in the Rabbit; there is nothing here visible, but a collection 

 of very transparent vesicles, surrounded by a mass of dark granules. In the 

 succeeding stage, represented in Fig. 2, some of the vesicles have enlarged, 

 and the granules immediately surrounding them have become developed into 

 cells. A more advanced condition is represented (on a smaller scale) in Fig. 

 3 ; in which a distinct spot (b] is seen on the central vesicle (a), marking it as 

 the Germinal Vesicle ; whilst many of the granules surrounding it have be- 

 come cells, and have taken-on a very regular arrangement. After a time, a 

 membrane forms around each cluster of granules, separating it from the stroma 

 of the ovarium ; this is the Ovisac. At a later period, a separation takes place 

 between the inner and outer portions of the mass of granular matter, included 

 between the ovisac and the germinal vesicle ; and the separation is completed 

 by the development of a membrane, which envelopes the inner stratum. This 

 stratum becomes the Yolk, and includes most of the oil-particles which pre- 

 viously existed within the ovisac ; whilst the portion of the granular mass, 

 exterior to this, gives origin in Mammalia to certain structures of a very pe- 

 culiar character, which seem to be concerned in the liberation of the ovum 

 from the Graafian follicle or Ovisac. The appearance of the Human Ovisac 

 and its contents is seen in Fig. 4. The granules immediately surrounding 

 the Ovum assume the appearance of cells ; and these unite to form a sort of 

 membrane, to which the name of tunica granulosa has been given. This is 

 seen at t g (Fig. 7). The granules lining the Ovisac also combine themselves 

 into a membranous structure ; to which Dr. Barry has given the designation 

 of membrana granulosa (g g, Fig. 6). These are connected by four band- 

 like extensions of the same cellulo-membranous structure, which seem to sus- 

 pend the ovum in its place ; and these are called retinacula (r r, Figs. 6" and 

 7). The space between the Tunica Granulosa and the Membrana Granulosa, 

 which is not occupied by the Retinacula, is filled with fluid, in which few or 

 no cells can be seen. The uses of this structure, so far as they are apparent, 

 will be described, when the processes by which the Ovum escapes from the 

 Ovary are detailed. 



907. The Ovisac does not form the entire structure which has been described 

 as the Graafian follicle ; for this consists of two layers, of which the inner 

 one is the true Ovisac, whilst the outer results from a thickening and conden- 

 sation of the surrounding layer of the Stroma of the Ovarium. It is the outer 

 layer only which is vascular ; the inner presents no trace of structure ; and 

 the increase of the ovum must take place by simple imbibition, through it, of 

 the supply of nutritive matter brought into contact with its exterior. The 

 Ovarium may be seen, even in the fetal animal, to contain immature Ova ; in 

 which the several parts can be clearly distinguished. At a later period, how- 

 ever, the number of Ova greatly increases ; and the development of some ad- 

 vances, whilst others degenerate. According to the recent valuable inquiries 

 of Dr. Ritchie,* it appears that, even during the period of childhood, there is a 

 continual rupture of Ovisacs, and discharge of Ova, at. the surface of the 



* London Medical Gazette, 1844. 



