i6o4 A MANUAL OF ANATOMY 



under a high power of the microscope, it presents very delicate 

 striae, which radiate across its breadth, and from this circumstance 

 it is known as the zona radiata. These striae are regarded as minute 

 pores or passages. 



External to the zona radiata there are several layers of cells, which 

 are disposed in a radiating manner and constitute the corona 

 radiata. These cells, like the zona radiata, are derived from the 

 discus proligerus within the Graafian follicle, and the innermost 

 cells send processes through the pores of the zona radiata to the 

 cytoplasm of the ovum. 



The cell-body, as in an ordinary cell, consists of cytoplasm 

 (ooplasm), and this presents the usual reticulum or spongioplasm, 

 the meshes of which are occupied by hyaloplasm. The ooplasm 

 constitutes the vitellus or yolk. Embedded in it there are several 

 fat-globules and albuminoid granules. These granules constitute 

 the denioplasm or nutritive yolk. According to some authorities, 

 the vitellus contains, in the earlier stages, an attraction-sphere and 

 centrosome, situated close to the nuclear membrane. 



The nucleus represents the germinal vesicle, and constitutes the 

 essential part of the ovum. As will be presently described, it forms 

 the mature ovum or female pronucleus, after extrusion of the two 

 polar bodies. It is a large spherical body, situated at first at the 

 centre of the ovum, but subsequently becoming eccentric. Its 

 diameter is about -^-^ inch, and it consists of the following parts : 



1. Nuclear membrane. 3. Karyoplasm. 



2. Nuclear reticulum. 4. Nucleolus. 



The nuclear membrane is well marked, and is formed by the 

 chromatin and linin of the nuclear reticulum. The nuclear reticu- 

 lum resembles that of a typical cell. 



The karyoplasm occupies the meshes of the nuclear reticulum. ■ 



The nucleolus represents the germinal spot. 



Oogenesis. 



Oogenesis is the begetting of ova. Each ovum is developed from 

 the germinal epithelium, the remnant of which epithelium covers the 

 adult ovary. The original source of the ovum is known as the 

 primordial germ-cell. These cells undergo many mitotic divisions, 

 and from the last generation oogonia are developed, which corre- 

 spond to the spermatogonia of the male. These oogonia divide by 

 mitosis, and give rise to primary oocytes. Each primary oocyte 

 represents the ovum as it leaves the Graafian follicle, and it corre- 

 sponds to a male primary spermatocyte. In the process of develop- 

 ment each primary oocyte undergoes two mitotic divisions, one 

 after the other, both of which are unequal. In the first division 

 the primary oocyte (mother-cell) extrudes the first polar body, and 

 then it becomes a secondary oocyte, which corresponds to a secondary 

 spermatocyte of the male. In other words, thq primary oocyte 



