512 THE REPRODUCTIVE SYSTEM 



tered as small round chromatic granules, which occur throughout the 

 cytoplasm. The physiological interpretation of these bodies is uncertain. 



The NUCLEUS of the ovum is a large spheroidal vesicle, the volume 

 and distribution of whose chromatin is subject to great variation. Chro- 

 matin is present in greatest amount during the period of most active cell 

 growth, in which the cytoplasm of the ovum is enormously increased in 

 volume. At this time the nucleus often appears as a solid mass of chro- 

 matin. Later the chromatin is diminished in volume, portions of its sub- 

 stance being possibly extruded into the surrounding cytoplasm; the 

 nucleus then acquires a characteristic vesicular appearance. 



The nuclear membrane is sharply defined and is at most times 

 prominent, except, as in other cells, during mitosis, a process which 

 marks the final maturation of the germ cell. The nuclear matrix or 

 nuclear sap abounds in the vesicular type of nucleus and the chromatin 

 is scattered in small particles which adhere to the inner surface of the 

 nuclear wall or to the. delicate achromatic linin threads. 



Each ovum as a rule contains a single nucleus (germinal vesicle), 

 though occasionally two nuclei occur. The latter condition is presumed 

 to arise either by the fusion of two ova within a single follicle or from 

 incomplete cell division during development. 



Each nucleus, during its vesicular stage, usually contains a single 

 nucleolus (germinal spot), which forms a spherical mass of chromatin, 

 situated, like the nucleus itself, eccentrically rather than centrally. The 

 staining properties of the nucleoli vary remarkably. Usually they take 

 the basic (nuclear) dyes to a greater or less depth; occasionally they 

 exhibit an allinily for the acid (cytoplasmic) dyes; still other nuclei take 

 a metachromatic or irregular tint with the ordinary nuclear stains. 

 Many nuclei even in the absence of mitosis contain no nucleolus. 



In the development of the ovum from the germinal epithelium, whose 

 cells from their homology with the spermatogonia have been termed 

 oiigonia, there occur several mitoses which result in so-called oocytes; 

 these later develop into the mature ovum. At about the time of its 

 extrusion from the ripe follicle a final series of mitoses occur, which dis- 

 tinguish the maturation of the ovum. In this process there is a series of 

 two mitoses which result in the appearance of the polar bodies and pro- 

 duce a reduction in the number of chromosomes to one-half the number 

 which is characteristic of the somatic cells. By the first mitosis the 

 cell produces what may he- termed a daughter ovum, together with the 

 first /Kiltir Ixnlij. a minute cell of insignificant size. A second mitosis 

 ensues giving origin to the mature or it in. or oo't'ul and the second polar 



