514 THE HUMAN BODY 



These are lined by cells, and each contains a single ovum. Jn a 

 woman of child-bearing age there will be found also, deeper in, 

 larger follicles (7, 8, 9, Fig. 153), their cavities being distended, 

 during life, by liquid ; in these the essential structure may be more 

 readily made out. Each has an external fibrous coat constituted 

 by a dense and vascular layer of the ovarian stroma; within this 

 come several layers of lining cells (9, a, Fig. 153) constituting the 

 membrana granulosa. At one point, b, the cells of this layer are 

 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. 154) is about 0.2 mm. (^~ 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, here named 

 the germinal vesicle and possessing a nu- 

 cleolus or germinal spot. The main bulk of 

 . b the vitellus or yolk consists of highly re- 

 fracting spheroidal particles of nutritive 

 matter (deutoplasm) embedded in and 

 concealing a true protoplasmic reticulum. 

 In the eggs of birds and reptiles the deuto- 

 plasm is in very large amount and forms 

 FIG. 154. A human nearly all of the yolk, the protoplasm being 



ovum; somewhat diagram- _ 



matic. a, zona pellucida; for the most part aggregated around the 

 germinal vesicle at a small area on one side 



of karyopiasm and a mi- o f the yolk. It is in this area that new 



cleolus or germinal spot. 



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 



