512 THE EEPEODUCTIVE SYSTEM 



tered as small round chromatic granules, which occur throughout the 

 cytoplasm. The physiological interpretation of these bofiies 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 qf 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, usual 

 nucleolus (germinal spot), which forms a spherical 

 situated, like the nucleus itself, eccentrically rather 

 staining properties of the nucleoli vary remarkably] 



contains a single 

 mass of chromatin, 

 lan centrally. The 



Usually they take 



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

 exhibit an affinity 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 spermatogoi}ia have been termed 

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

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

 extrusion from the ripe follicle a final series of mftoses 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 be termed a* daughter ovum, together with the 

 first polar body, a minute cell of insignificant /size. A second mitosis 

 ensues giving origin to the mature ovum or oojtid and the second polar 



