THE OVUM 397 



scattered as small round chromatin 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 vol- 

 ume and distribution of whose chromatin is subject to great varia- 

 tion. Chromatin 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 chromatin. Later the chromatin is 

 diminished in volume, portions of its substance 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 con- 

 dition is presumed to arise either by the fusion of two ova within 

 a single follicle or from incomplete cell division during develop- 

 ment. 



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 re- 

 markably. 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 ir- 

 regular 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 oogonia, there occur several mitoses which result in 

 so-called oocytes ; these later develop into the complete ovum. At 

 about the time of its extrusion from the Graafiah follicle a final 

 series of mitoses occur, which distinguish 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 produce a reduc- 



