8 



STKUCTUKE OF OVAEIAN OVUM 



moderate power of the microscope a faint radial striation can generally be made 

 out, but this is seen more distinctly in ova which have been fixed by reagents, 

 and more especially in sections. On this account, and especially since a similar 

 radially striated membrane forms a characteristic part of the investment of the 

 ovum in many animals belonging to widely different classes, it is usual, in place 

 of the name zona pellucida, which has been exclusively used to designate this 

 investment in mammals, to employ the more general term zona radiata. (Waldeyer, 

 1870), or to speak of it simply as the striated membrane of the ovum. 



FIG. 15. HUMAN OVUM EXAMINED FRESH IN THE LIQUOR FOLLICULI. (Waldeyer.) 



The zona pellucida is seen as a thick clear girdle surrounded by the cells of the corona radiata. 

 The egg itself shows a central granular deutoplasmic area and a peripheral clear layer, and encloses 

 the germinal vesicle, in which is seen the germinal spot 



The zona radiata of the mammalian ovum is sufficiently tough to prevent the 

 escape of the contents of the ovum, even when subjected to a considerable 

 amount of pressure. If, however, the pressure be excessive, the tunic splits, 

 and the soft contents are extruded (fig. 16). It has, however, to be particularly 

 noticed that part of the contents remain, at any rate in all but the riper 

 oocytes, attached to the zona. The striae are believed to be minute pores ; and 

 it has been shown by Flemming, Heape, Retzius, and Ebner, that they are 

 occupied by processes of the cells of the corona radiata. This name is given 



