Fig. 250. 



521 



Section of Graafian vesicle. 



Ovum. 



Section of ovary. 



Fig. 250, ovum of the sow : 1, germinal spot ; 2, germinal vesicle ; 

 3, yolk ; 4, zona pellucida ; 5, discus proligerus ; 6, adherent granules or 

 cells. (Barry.) 



The diameter of the human ovum varies from the yj-^ to the -^g- of 

 an inch. It consists of an exterior transparent membrane, Description of 

 the Y^fr o" f an i nc k * n thickness, which, when compressed the ovum - 

 for the purpose of examination, appears like a diaphanous circle, and hence 

 called zona pellucida. Within this zone, and inclosed by it, is the yolk, 

 a granular material suspended in or intermingled with fluid, the granules 

 being of different sizes ; those near the pellucid zone are the largest. For 

 the most part, the yolk consists of albumen and oil globules. ' Its condi- 

 tion, as regards liquidity, varies in different animals ; in some it is al- 

 most a soft solid, so that, when water percolates through the zona pellu- 

 cida, it isolates the yolk by surrounding it on all sides, and parting it off 

 from the zone. Within the substance of the yolk is a distinct cell, the 

 germinal vesicle, which gradually makes its way from the interior to the 

 place of peritoneal contact. As it advances to perfection, it consists of a 

 delicate spherical membrane containing a liquid, in which granules are 

 suspended. Upon that portion of it nearest to the place of peritoneal 



contact is its nucleus, the germinal 

 spot, about the - 8 * of an inch 

 in diameter, and consisting of yel- 

 low granules. 



Fig. 251, diagram of a Graa- 

 fian vesicle and ovum : 1, stroma 

 of ovary ; 2, 3, external and in- 

 ternal tunics of the Graafian ves- 

 icle ; 4, cavity of vesicle ; 5, thick 

 tunic of the ovum or yolk-sac ; 6, 

 the yolk ; 7, the germinal vesicle ; 

 8, the germinal spot. 



The most mature ova are near- 

 est the surface of the ovary, but 

 Diagram of Graafian vesicle. are separated from its peritoneum 



251. 



