308 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



stratum granulosum and most of the discus pro- 

 ligerus remaining behind in the ovary. 



The Ovum. The ovum has already been men- 

 tioned as a large spherical cell with a large accumu- 

 lation of food material. It measures 0.2 mm. in 

 diameter and is barely visible to the naked eye. 

 When examined under the microscope, even before 

 the rupture of its follicle, it is found encircled by a 

 clear substance called the zona pellucida, which upon 

 a closer examination may be found to contain trans- 

 verse striations, hence it has also been called zona 

 radiata. It is not uncommon for a few of the epi- 

 thelial cells of the dis- 

 cus proligerus to re- 

 main attached to this 

 layer, if so they are 

 called the corona radi- 

 ata. The zona pelluci- 

 da is a secretion from 

 Fig. 224. The ovum. the adjoining cells, and 



one theory of the trans- 

 verse striations is that they are produced by minute 

 cellular processes from the cells that form the corona 

 radiata; that is, the first row of epithelial cells in- 

 vesting the ovum. It is affirmed by some that these 

 processes are in direct communication with the sub- 

 stance of the ovum and are the means by which 

 elaborated food material is contributed to the latter. 

 The zona pellucida no doubt serves to strengthen 

 the delicate ovum after its expulsion from the ovary. 

 The transverse striae in this membrane may serve a 

 further purpose as primitive channels for the entrance 



