116 THE PHYSIOLOGY OF REPRODUCTION 



account of which was published in his famous monograph 

 Eier stock und Ei. 1 He found that in the chick, on the fourth 

 day of development, the ccelomic epithelium which covers 

 the inner surface of the Wolffian body became differentiated 

 from the tissue surrounding it, the cells being relatively large 

 and cuboidal in shape. A little later he observed that the cells 

 had multiplied to such an extent as to give rise to a distinct 

 elevation. In this way the germinal epithelium was formed, 

 and this marked the site of the future ovary. The mesoblast 

 underlying the germinal epithelium is described as growing 

 upwards among the cells of the latter, and so giving rise to the 



G.E. 



FIG. 21. Section through ovary of pig embryo. (From Williams' 

 Obstetrics, Appleton & Co.) 



G.E., germinal epithelium ; S., stroma. 



appearance of those germinal ingrowths or " egg-tubes," which 

 were described by Pfliiger. 



The cells of the germinal epithelium are thus divided by 

 mesoblast into clusters of " egg-nests " which contain the pri- 

 mordial ova, as Waldeyer has shown. As a result of this process 

 two zones of tissue are formed in the future ovary. The outer 

 or cortical zone consists of clusters of cells derived from the 

 germinal epithelium, with mesoblastic processes in between 

 them. The inner or medullary zone is composed at first entirely 

 of mesoblast, which gives rise to the vascular tissue and stroma 

 of the ovary. 



The majority of investigators, including Balfour, 2 Schafer, 3 



1 Waldeyer, Eierstock und Ei, Leipzig, 1870. 



2 Balfour, " Structure and Development of the Vertebrate Ovary," Quur. 

 Jour. Micr. Science, vol. xviii., 1878. 



* Schafer, " On the Structure of the Immature Ovarian Ovum,"&c., Proc. 

 Roy. Soc. , vol. xxx., 1880. 



