REPRODUCTION 571 



heaped up, forming the discus proligerus, which projects into the 

 liquid filling the cavity of the follicle. Buried among the cells of 

 the discus proligerus the ovum, c, lies. 



The Mammalian Ovum. As the Graafian follicles enlarge the 

 ova grow but not proportionately, so that they occupy relatively 

 less of the cavities of the larger follicles : the cells of the discus pro- 

 ligerus probably elaborate food for the egg-cell from material de- 

 rived from the blood-vessels which form a close network around 

 most of each enlarging Graafian follicle and transude crude nutri- 

 tive matter into the liquid filling most of the follicle. The fully 

 formed ovum (Fig. 156) is about 0.2 mm. (y|o inch) in diameter: 

 it has a well-marked outer coat or sac, a, the zona radiata, zona 

 pellucida or vitelline membrane, surrounding a very granular cell- 

 body or vitellus, b, in which is a conspicuous nucleus, c, with its 

 characteristic network of chromatin. The 

 main bulk of the vitellus or yolk consists of 

 highly refracting spheroidal particles of 

 nutritive matter (deutoplasm) embedded 

 in and concealing a true protoplasmic 

 reticulum. In the eggs of birds and reptiles 

 the deutoplasm is in very large amount 

 and forms nearly all the yolk, the proto- 

 plasm being for the most part aggregated FlG - 156. A human 



ovum; somewhat diagram- 



around the nucleus at a small area on one matic. a, zona radiata; 6, 

 side of the yolk. It is in this area that new viteUu3 or yolk; c ' m 

 cell-formation occurs and the embryo is built up, the rest of the 

 yolk being gradually absorbed by it; such eggs are known as 

 mesoblastic or partly dividing eggs. In all the higher mammalia 

 the deutoplasm is relatively sparse and tolerably evenly mingled 

 with the protoplasm, and the whole fertilized ovum divides to 

 form the first cells of the embryo: such eggs are named holoblastic. 

 Ovulation. From puberty, during the whole child-bearing 

 period of life, certain comparatively very large Graafian follicles 

 may nearly always be found either close to the surface of the ovary 

 or projecting on its exterior. These, by accumulation of liquid 

 within them, have become distended to a diameter of about 4 mm. 

 (i inch); finally, the thinned projecting portion of the wall of the 

 follicle, which differs from the rest in containing few blood-vessels, 

 gives way and the ovum is discharged, surrounded by some cells of 



