THE UNIMPEEGXATED OVUM. 719 



The ovum is a minute spherical body situated, in 

 immature follicles, near the centre; but in those nearer 



maturity, in contact with the 



, . Fig. 208. * 



membrana granulosa at that 



part of the follicle which 

 forms a prominence on the 

 surface of the ovary. The 

 cells of the membrana granu- 

 losa are at that point more 

 numerous than elsewhere, 

 and are heaped around the 



ovum, forming a kind of granular zone, the discus proliyerm 

 (fig. 208). 



In order to examine an ovum, one of the Graafian 

 vesicles, it matters not whether it be of small size or 

 arrived at maturity, should be pricked, and the contained 

 fluid received upon a piece of glass. The ovum then, being 

 found in the midst of the fluid by means of a simple lens, 

 may be further examined with higher microscopic powers. 

 Owing to its globular form, however, its structure cannot 

 be seen until it is subjected to gentle pressure. 



The human ovum is extremely small, measuring, accord- 

 ing to Bischoff, from ~^ to T |- 5 - of an inch. Its external 

 investment is a transparent membrane, about j-Vu- of an 

 inch in thickness, which under the microscope, appears as 

 a bright ring (fig. 209), bounded externally and internally 

 by a dark outline : it is called the zona pellucida , or vitelline 

 membrane. It adheres externally to the heap of cells con 

 stituting the discus proligerus. 



Within this transparent investment or zona pellucida, 



* Fig. 208. Section of the Graafian vesicle of a Mammal, after Von 

 Baer. I. Stroma of the ovary with blood-vessels. 2, Peritoneum. 

 3 and 4. Layers of the external coat of the Graafian vesicle. 5. Mem- 

 brana granulosa. 6. Fluid of the Graafian vesicle. 7. Granular zone, 

 or discus proligerus, containing the ovum (8). 



