850 



DEVELOPMENT OF THE OVA. 



as the original 



Yelk. 



Zona 

 pellucida. 



The zona pellucida (figs. 640, 641, V, Z), to which cells the Graafian follicles are often in- 

 herent isTcuTicular mentbrane formed secondarily by the follicle (Pfluger) Accor, img to \an 

 Ikneden it is lined by a thin membrane next the vitellus, and he regards the thin membrane 

 rinal cell-membrane of the ovum. The fine radiating strne m the zona are said to be 

 _ t .. ,,. due to the existence of nume- 



Cells of discus proUgerus. ^ ^^ ^^ % 



Sehleri). It is still undecided 

 whether there is a special 

 micropyle or hole for the 

 entrance of the spermatoza. 



A micropyle has been 

 observed in some ova (holo- 

 thurians, many fishes, mus- 

 sels). The ova of some 

 animals (many insects, e.g., 

 the flea) have porous canals 

 in some part of their zona, 

 and these serve both for the 

 entrance of the spermatozoa 

 and for the respiratory ex- 

 changes in the ovum. 



The development of 

 the ova takes place in the 

 following manner: The 

 surface of the ovary is 

 covered with a layer of 

 Fig- 640. cylindrical epithelium 



Ripe ovum of rabbit. the so-called " germ-epi- 



thelium" and between these cells lie somewhat spherical "primordial ova" 

 (fig. G41, I, a, a). The epithelium covering the surface dips into the ovary 

 at various places to form " ovarian tubes " (fig. 683). These tubes, from and 

 in which the ova are developed (Waldeyer), become deeper and deeper, and they 

 contain, in their interior, large single spherical cells with a nucleus and a nucleolus, 

 and other smaller and more numerous cells lining the tube. The large cells are 

 the cells (primordial ova) that are to develop into ova, while the smaller cells are 

 the epithelium of the tube, and are direct continuations of the cylindrical epithelium 

 on the surface of the ovary. The upper extremities of the tubes become closed, 

 while the tube itself is divided into a number of rounded compartments snared 

 off, as it were, by the ingrowth of the ovarian stroma (I, c). Each compartment 

 so snared off usually contains one, or at most two, ova (IV, o, o), and becomes 

 developed into a Graafian follicle. The embryonic follicle enlarges, and fluid 

 appears within it ; while its lateral small cells become changed into the epithelium 

 lining the Graafian follicle itself, or those of the membrana granulosa. The cells 

 of the membrana granulosa form an elevation at one part the discus proligerus 

 by which the ovum is attached to the membrana granulosa. The follicles are at 

 first only 0*03 mm. in diameter, but they become larger, especially at puberty. 

 [The smaller ova are near the surface of the ovary, the larger ones deeper in its 

 substance (fig. 639).] When a Graafian follicle with its ovum is about to ripen 

 (IV), it sinks or passes downwards into the substance of the ovary, and enlarges at 

 the same time by the accumulation of fluid the liquor folliculi between the 

 tunica and membrana granulosa. It is covered by a vascular outer membrane 

 the theca folliculi which is lined by the epithelium constituting the membrana 

 granulosa (IV, g). When a Graafian follicle is about to burst, it again rises to 

 the surface of the ovary, and attains a diameter of 1*0 to 1'5 mm., and is now ready 

 to burst and discharge its ovum. [The tissue between the enlarged Graafian 

 follicle and the surface of the ovary gradually becomes thinner and thinner and less 

 vascular, and at last gives way, when the ovum is discharged and caught by the 



