THE FEMALE REPRODUCTIVE ORGANS 



511 



climacteric the remaining follicles degenerate and the process of ovulation 

 gradually ceases. 



We shall now discuss the structure of the ovum or female germ cell 

 and shall then successively trace its development and maturation, the 

 formation of its vesicular (Graafian) follicle, the rupture of the follicle, 

 and the subsequent history of the corpus luteum. 



The Ovum. The ovum is a spherical cell of large size (200 to 

 300 /*). When fully developed it is surrounded by a thick layer of 

 exoplasm, the zona pellucida, which is probably 

 derived from the cytoplasm of the follicular epi- 

 thelium by which the j ovum is closely invested. 

 The ovum itself consists of a mass of cytoplasm, 

 the vitellus, and a large vesicular nucleus or 

 germinal vesicle, within which is frequently a 

 single prominent nucleolus or germinal spot. The 

 cytoplasm of the- mature ovum is inclosed by a 

 very delicate cell membrane, known as the vitelline 

 membrane^w^ch is noT demonstrable in the prim- 

 itive ova of the younger follicles. 



The CYTOPLASM of the ovum at first appears 

 finely reticular, but as its development advances 

 a fatty material is deposited within its nfeshes, 

 usually in the form of minute irregular spheroids, 

 by the accumulation of which the reticulan cyto- 

 plasm is in great part replaced by a granujpfatty 

 mass .of faint yellowish color known as laeuto- 

 plasm. Frequently this metamorphosis is not 



quite complete, a remnant of the original cytoplasm persists ij^neath the 

 vitelline membrane and in the vicinity of the nucleus. 



Numerous cytoplasmic structures have been described in, tjhese cells, 

 chief among which are the accessory nucleus (Nebenkern), and the yolk 

 nucleus (Dotterkern). The accessory nuclei, occasionally chromatinic 

 and therefore basophil, more frequently stain with cytoplasmic dyes and 

 are at times attached, at other times separate from the true nucleus. 

 They are more probably remnants of mitotic spindles. The yolk nuclei 

 of mammalian ova most frequently take the form of crescentic masses 

 of lightly staining chromatin ('chrornidia') which partially surround the 

 nucleus, forming a so-called nuclear cap. They are often found in 

 various stages of disintegration, and the fragments may be transported 

 to the peripheral portions of the cytoplasm, or may be irregularly scat- 



FIG. 451 .OVUM, CON- 

 TAINING A YOLK 

 NUCLEUS (DOTTER- 

 KERN) AT THE LEFT 

 AND ABOVE THE NU- 

 CLEUS. 



The peripheral nuclei 

 are derived from the 

 adjacent stroma. Iron- 

 hematoxylin. Highly 

 magnified. (After von 

 Skrobansky.) 



