THE OVUM. 



13 



parts of a typical animal cell, and, in addition, each has special peculiarities which 

 differentiate it both from the germ-cells of the opposite sex and also from ordinary 

 animal cells. Therefore the oocyte and the spermatocyte must be considered 

 separately ; but before this is done it must be noted that each oocyte I and each 

 spermatocyte I is capable of producing only four descendants. The mitotic cell 

 divisions by which the descendants are produced are called the maturation divisions, 

 and they result, in the case of the oocyte, in the formation of one large functional 

 cell the mature ovum, and three small impotent cells the polar bodies ; whilst in 

 the case of the spermatocyte the four descendants are of equal size and each 

 becomes transformed into a presumably potent spermatozoon. 



THE OVUM. 



An ovum presents all the characteristic structural features of an animal cell, 

 but it is peculiar on account of its relatively large size, the large size of its nucleus, 

 and the possession of an investing membrane, the oolemma. As the young ova or 

 oocytes of the first order enter upon their 

 period of growth, each is enclosed by a 

 single layer of special cells, the stratum 

 granulosum, which constitute, together 

 with the oocyte, a primary ovarian follicle 

 (O.T. Graafian follicle). The cells of the 

 stratum granulosum multiply rapidly until 

 they form a layer, several cells thick. 

 At the same time, the oocyte increases 

 in size and becomes surrounded by the 

 membrane, called the oolemma, which 

 intervenes between it and the innermost 

 cells of the stratum granulosum. 



Whilst the growth of the oocyte and 

 the thickening of the oolemma are still 

 proceeding, a fluid-filled cavity appears in 

 the stratum granulosum. Whether the 

 cavity is due to the imbibition of fluid 

 or to the dehiscence of the cells of the 

 stratum granulosum is still uncertain, but, 



FIG. 11.- 



-THE OVUM AND ITS COVERINGS 

 ( Diagrammatic) . 



after its appearance, the cavity with its The corona radiata) which com p letely surrounds the 



ovum, is only represented in the lower part of 

 the figure. 



5. Vitellus or Yolk. 



6. Nucleus (germinal vesicle). 



7. Nucleolus (germinal spot). 

 . Oolemma (zona pellucida). 8. Nuclear membrane. 



surrounding walls and the enclosed oocyte 

 is spoken of as a vesicular ovarian follicle. 

 The cavity of the vesicular follicle gradu- J- corona radiata. 



* & 2. Granular layer. 



ally increases, and, as it grows, it separates 3. viteiime membrane. 

 the oocyte and the cells of the stratum 4 

 granulosum immediately around the oocyte from the remainder of the cells of the 

 stratum, except in a small area where the two parts of the stratum granulosum 

 still remain in direct continuity. When this condition is attained the cells of 

 the stratum granulosum which immediately surround the oolemma are spoken 

 of as the ovular cumulus ; they enclose the oocyte, and, together with it, they 

 form a bold promontory which projects into the cavity of the follicle. 



When its full growth is attained each oocyte I is a comparatively large cell, 

 which measures 200//. in diameter. It consists of a cell body which is surrounded 

 by a definite enclosing membrane, the oolemma, and it contains (1) a nucleus, (2) 

 a centrosome, (3) numerous granules called deutoplasmic or yolk granules, and 

 (4) mitochondria. 



The Oolemma. The limiting membrane or oolemma is also called the zona 

 pellucida, on account of its appearance under low magnifying powers, and the zona 

 striata, because, under certain conditions, radial striae are seen in it when it is 

 highly magnified. It is a strong, elastic membrane, which not only protects the 

 oocyte from pressure, but probably also prevents the impregnated oocyte or zygote 



