720 GENERATION AND DEVELOPMENT. 



The ovum is a minute spherical body situated, in im- 

 mature follicles, near the centre; but in those nearer 

 p. 2QI * maturity, in contact with the 



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 

 proligerus (fig. 201). 



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 -^ of an inch. Its external 

 investment is a transparent membrane, about ^-sW ^ an 

 inch in thickness, which under the microscope, appears as 

 a bright ring (fig. 202), 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. 201. 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). 



