656 THE HUMAN BODY. 



each about 0.25 mm. ( T J incli) in diameter, will be found 

 imbedded in it near the surface. These are lin^d by cells, and 

 each contains a single ovum. In a woman Sf child-bearing 

 age there will be found also, deeper in, larger follicles (7, 8, 

 9, Fig. 189), 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, &, Fig. 189) constituting 

 the membrana granulosa. At one point, #, 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 proligerus probably elaborate food for the egg cell 

 from material derived from the blood-vessels which form a 

 close network around most of each enlarging Graafian follicle 

 and transude crude nutritive matter into the liquid filling 

 most of the follicle. The fully formed 

 ovum (Fig. 19Q) is about 0.2 mm. 

 (yi-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 granu- 

 lar cell-body or viteiius, b, in which is a 

 conspicuous nucleus, c, here named 

 the germinal vesicle and possessing 

 FIG. 190. A human ovum; a nuc leolus or germinal spot. The 



somewhat diagrammatic, a, ..... 



zona peiiucida; b, viteiius; c, zona pellucida exhibits distinct radial 



germinal vesicle, with distinct . 



reticuium of karyopiasm and markings which probably are due to 

 flue tubes traversing it. The main bulk 



of the viteiius or yelk consists of highly refracting spheroidal 

 particles of nutritive matter (deutoplasm) imbedded in and 

 concealing a true protoplasmic reticuium. In the eggs of birds 

 and reptiles the deutoplasm is in very large amount and forms 

 nearly all of the yelk, the protoplasm being for the most part 

 aggregated around the germinal vesicle at a small area on one 

 side of the yelk. It is in this area that new cell-formation 

 occurs and the embryo is built up, the rest of the yelk being 

 gradually absorbed by it: such eggs are known as mesoblastic 



