450 THE HUMAN EMBRYO. 



.already described in the rabbit that a detailed account will be 

 unnecessary. 



The germinal epithelium. In embryos of about the fifth 

 week, the genital ridges appear, as a pair of longitudinal bands 

 along the dorsal wall of the abdominal cavity, close to the inner 

 borders of the Wolffian bodies. The ridges, which at first are 

 merely caused by the epithelial cells becoming columnar in place 

 of squamous in shape, rapidly increase in thickness, partly owing 

 to active division of the cells of the germinal epithelium 

 covering the ridge, and partly owing to ingrowth of connective 

 tissue along their axes. 



At an early age, an intimate relation is established between 

 each genital ridge and the corresponding Wolffian body, a 

 number of rod-like outgrowths arising from the Malpighian 

 bodies of the Wolffian body, and growing into the substance of the 

 genital ridge. These rods subsequently become hollow, and form 

 the so-called tubuliferous tissue of the ovary. This lies, at first, 

 close beneath the germinal epithelium, but soon withdraws into 

 the deeper part of the ovary ; it has nothing to do with the 

 formation of the ova, and merely requires mention on account 

 of its great prominence during the early stages of develop- 

 ment. 



The germinal epithelium gives rise to the ova in much the 

 same way as in the rabbit. In its earliest stages it is a single 

 layer of columnar epithelium cells, with large nuclei, the cells 

 measuring on an average about 0*014 mm. in length by 0-007 

 mm. in width. 



By division of its cells, the germinal epithelium rapidly 

 increases m thickness. The surface cells remain columnar, but 

 the deeper cells, which are spherical or polygonal in shape, 

 grow down into the connective-tissue stroma as irregular 

 branching rods of cells, the egg columns (Fig. 171). By further 

 growth inwards of the egg columns, accompanied by active 

 growth outwards of the vascular connective tissue, the structure 

 of the ovary rapidly becomes more complicated. In place of the 

 original arrangement, of a layer of epithelial cells clothing a 

 central connective-tissue core, there is now (Fig. 171) a super- 

 ficial layer of columnar epithelium, a, beneath which is a reti- 

 cular framework of connective tissue, the meshes of which are 



