THE EGG. 345 



In a rabbit embryo of about the eleventh day, a pair of 

 ridge-like thickenings of the peritoneal epithelium appear, close 

 to the mesentery, arid along the inner sides of the Wolffian 

 bodies. These genital ridges (Fig. 165, GR) constitute the earliest 

 stage in the development of the ovaries, and are at first caused 

 merely by an alteration in the shape of the epithelial cells of the 

 peritoneum, which, elsewhere flattened, become here columnar. 



The genital ridges soon become more prominent, partly 

 through an increase in the thickness of the epithelium, which 

 becomes two or three cells deep ; and partly through the in- 

 growth into each ridge of an axial core of connective tissue. 



The genital ridge (Fig. 165) lies very close to the Wolffian 

 body ; and, about the fourteenth day, solid columns of cells grow 

 out from the Malpighian bodies of the anterior end of the 

 Wolffian body into the ridge. These columns of cells form 

 what is called the tubuliferous tissue of the ovary ; in the early 

 stages they occupy almost the whole of the axial part of the 

 genital ridge, but as development proceeds they gradually 

 become less conspicuous, and withdraw more or less completely 

 from the ridge. They have nothing whatever to do with the 

 formation of the ova. 



In a rabbit embryo of the eighteenth day the genital ridges 

 have grown considerably, and project into the body cavity as a 

 pair of longitudinal folds, close to the attachment of the 

 mesentery to the dorsal body wall. Each ridge is covered by 

 the germinal epithelium, which consists of two or three layers of 

 cells, the outermost of which are columnar in shape, while the 

 more deeply placed ones are more or less spherical. The central 

 part, or core, of the genital ridge is made up almost entirely of 

 the tubuliferous tissue, with numerous blood-vessels and a small 

 amount of connective tissue. 



By the twenty-second ' day the germinal epithelium has 

 increased considerably in thickness ; and among the cells of 

 which it consists are some of larger size than the rest, these 

 larger cells being the primitive ova. By the twenty-eighth day, 

 i.e. about two days before birth, the genital ridges are still 

 larger ; the germinal epithelium covering their surface is much 

 thicker than before, and its deeper layers are honeycombed, or 

 broken up into irregular columns, by ingrowth of the vascular 

 connective tissue from below. 



