THE OVUM. 55 



like structure formed of trabeculae of germinal epithelium inter- 

 penetrated by vascular strands of stroma. The trabeculae of the 

 germinal epithelium form the egg-tubes of Pfliiger. 



With reference to the distribution of the stroma in the germi- 

 nal epithelium, it may be said in a general way that there is a 

 special layer close to the surface of the ovary, which, after the 

 formation of fresh ova has nearly ceased, completely isolates a 

 superficial layer of the germinal epithelium from the deeper and 

 major part of it. The superficial layer is frequently (but errone- 

 ously) regarded as constituting the whole of the germinal epi- 

 thelium. The layer of stroma below the superficial epithelium 

 forms in the mammalian ovary the tunica albuginea. As the 

 follicles are formed- in the trabeculae of germinal epithelium the 

 stroma grows in around them, and forms for each one of them a 

 special tunic. 



FIG. 1 8. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OF 



SCYLLIIIM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING IN 

 THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE. 



The adult ovaries differ in a corresponding manner to the em- 

 bryonic genital ridges as to the presence of a core of stroma. 

 The ovaries which are without such a core in the embryo, are 

 also without it in the adult, and are formed of a double layer of 

 tissue entirely derived from the germinal epithelium with its in- 

 growths of stroma, and composed, for the most part, of ova in 

 all stages of development. In the case of the other ovaries there 



