564 GENERATION AND DEVELOPMENT. 



which forms a prominence on the surface of the ovary. The 

 cells of the membrana granulosa are at that point more numer- 

 ous than elsewhere, and are heaped around the ovum, forming 

 a kind of granular zone, the discus proligerus (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 according 

 to Bischoff, from 5 |(j to ^-Q- of an inch. Its external invest- 

 ment is a transparent membrane, about ^QQ of an inch in 

 thickness, which under the microscope, appears as a bright 

 ring (Fig. 209), bounded externally and internally by a dark 



FIG. 208. Section of the Graafian vesicle of a Mammal, after Von Baer. 1. Stroma 

 of the ovary with bloodvessels. 2. Peritoneum. 3 and 4. Layers of the external 

 coat of the Graafian vesicle. 5. Membrana granulosa. 6. Fluid of the Graafian 

 vesicle. 7. Granular zone, or discus proligerus, containing the ovum (8). 



Fitt. 209. Ovum of the sow, after Barry. 1. Germinal spot. 2. Germinal vesicle. 

 3. Yelk. 4. Zona pellucida. 5. Discus proligerus. 6. Adherent granules or cells. 



outline : it is called the zona pellucida, or vitelline membrane. 

 It adheres externally to the heap of cells constituting the dis- 

 cus proligerus. 



Within this transparent investment or zona pellucida, and 

 usually in close contact with it, lies the yelk or vitellus, which 

 is composed of granules and globules of various sizes, imbed- 

 ded in a more or less fluid substance. The smaller granules, 

 which are the more numerous, resemble in their appearance, 

 as well as their constant motion, pigment-granules. The larger 

 granules or globules, which have the aspect of fat-globules, are 

 in greatest number at the periphery of the yelk. The number 

 of the granules is, according to Bischoff, greatest in the ova of 



