THE OVUM. 289 



Structure of the Ovum. 



The ripe ovum, as we have already indicated, lies in the 

 Graafian follicle, embedded in the mass of cells which con- 

 stitutes the discus proligerus. "Within the discus, surround- 

 ing the ovum, there seem to be two kinds of cells ; first, cells 

 evidently belonging to the Graafian follicle, and similar to 

 the cells in other parts of the membrana granulosa ; second, 

 a single layer of columnar cells, belonging to the ovum, 

 and probably concerned in the production of the proper 

 membrane of the ovum, the vitelline membrane. Regarding 

 the vitelline membrane as the external covering, we can see, 

 in the ovum, a clear, transparent membrane, a granular mass, 

 the vitellus, filling this membrane completely, a large, clear 

 nucleus, called the germinal vesicle, and a nucleolus, called 

 the germinal spot. 



The size of the ripe ovum, in the human subject and in 

 mammals, is about y^-y of an inch, and its form is globular. 



The external membrane is clear, apparently structureless, 

 quite strong and resisting, and measures about 2S * 00 of an 

 inch in thickness. As it forms a transparent ring in the mass 

 of cells in which the ovum is embedded, this is sometimes 

 called the zona pellucida. According to recent researches, it 

 seems that the primordial ovum has at first no special invest- 

 ing membrane ; as it develops, it presents, surrounding the 

 vitellus, a single layer of columnar cells ; at the deepest por- 

 tion of these cells, a homogeneous basement membrane is 

 gradually formed ; and the cells undergo a sort of cuticular 

 transformation, becoming finally the vitelline membrane. 1 



An important point, in this connection, is the question of 

 the existence of pores, or perforations in the vitelline mem- 

 brane. As we shall see farther on, there can be no doubt as to 

 the actual penetration of the spermatozoids through this mem- 

 brane, so that they come in contact with the vitellus ; and it is 



1 WALDEYER, in STRICKER, Manual of Human and Comparative Histology, 

 The New Sydenham Society, London, 1872, vol. ii., p. 177. 



