THE SEXUAL ELEMENTS OVUM AND SPERMATOZOON. 



11 



essentially the structure of a typical cell. Around the ovum and separated 

 fi :>m it by a narrow cleft the perivitelline space is the zona pellucida, a rather 

 thick, highly refractive membrane which shows radial striations. These 

 si iulions are probably due to the presence of minute canals which penetrate the 

 zona. It has been suggested that these canals serve for the passage of nutri- 

 ment to the ovum. Immediately outside of the zona pellucida the epithelial 

 cells of the Graafian follicle are arranged radially in one or two layers. These 



Zona 

 pellucida 



FIG. 5. From a section of a human ovum. Section taken from the ovary of a 12 year old girl. 

 The ovum lies in a large mature Graafian follicle and is surrounded by the cells of the "germ hill" 

 (the inner edge of which is shown in the upper left-hand corner of the figure). Photograph. 



constitute the corona radiata (Fig. 5). Some investigators have described a 

 thin, delicate mtelline membrane between the perivitelline space and the ovum. 

 Others have failed to observe this. 



The egg protoplasm, originally called the vitellus, differs from the pro- 

 toplasm of most cells in that it appears somewhat more opaque and coarsely 

 granular. This appearance is due to the fact that the ovum stores up within 

 itself food stuffs. These consist of fatty and albuminous substances which are 



