CHANGES IN THE OVARY 121 



generally be seen showing a very considerable protrusion, while in 

 some animals, such as the sow, the appearance of the ovary at this 

 time is not dissimilar to a hunch of grapes. 



A large Graafian follicle in a mature ovary contains the following 

 parts: Forming the outermost part of the wall and in continuity 

 with the ovarian stroma is the theca externa, which is a layer of 

 somewhat fibrous connective tissue. Within this is the theca interna, 

 which is less fibrous. The two thecse are only slightly modified 

 ovarian stroma. Within the theca interna is the epithelial wall, 

 which, in the very young follicles, consists of a single layer of cells 

 immediately surrounding the ovum. These, as already mentioned, 

 multiply rapidly (by mitotic division) and give rise to a layer many 

 cells deep, which, as the follicle increases in size, becomes divided 

 into two layers, the membrana granulosa lining the follicle, and the 



Fig. 34. — Young oocyte or egg surrounded by a single layer of follicular 

 epithelial cells. (From van der Stricht.) 



discus proligerus surrounding the ovum. The innermost cells of 

 the discus rest upon a thick, transparent, radially striated membrane 

 with a granular outer border. This is a zona radiata or zona pellucida. 

 The striated appearance is due to the presence of fine canals. Within 

 the zona, and immediately enclosing the ovum, another very thin 

 membrane can sometimes be made out. This is the vitelline mem- 

 brane. The membrana granulosa and discus proligerus are united 

 by one or more strands of follicular epithelial cells. A viscid fluid, 

 containing protein matter, collects between them and becomes gradu- 

 ally increased in quantity as the follicle continues to grow.^ 



* Occasionally a Graafian follicle may contain more than one ovum, but 

 this is abnormal. Such follicles have been described as occurring in the 

 rabbit's ovary by Honor6 ("Eecherohes sur FOvai'ie du Lapin," Arch, de 

 Biol., vol. xvii., 1901), and in the dog's ovary by Smyth ("An Unusual Graafian 

 Follicle," Biol. Bull., vol. xiv!, 1908). The latter writer states that one follicle 

 contained seven ova. He shows that the tendency to produce multiple ova 

 may be Jbereditary, and that it is apparently correlated with a high fertility. 

 Multiovular follicles have also been observed in pigs by Corner (The Cm-piis 

 Lutewm, etc., Carnegie Institute (Washington) Publication 222, 1915), and in 



