384 THE ORGANS 



During foetal life ihe kidneys are disLincUy lobulated, but after birth the 

 surface becomes quite smooth. 



The genital gland on each side appears on the mesial surface of the meso- 

 nephros as a thickening of a narrow band of the mesothelial lining of the body 

 cavity. The cells in the band become differentiated into two kinds — small 

 cuboidal cells which stain rather intensely, and larger spherical cells with clearer 

 cytoplasm and vesicular nuclei. The latter are the sex cells which are destined 

 to give rise to the ova in the female or the spermatozoa in the male. The 

 whole thickened band is known as the germinal epithelium. 



The cells of the germinal epithelium increase in number by mitosis and soon 

 become differentiated into two layers — a superficial layer which retains its 

 epithelial character and contains the sex cells, and a deeper layer which is des- 

 tined to give rise in part to the stroma of the genital gland. The elevation 

 caused by the increased number of cells projects into the body cavity as the 

 genital ridge. From the superficial layer containing the sex cells a number of 

 plugs or columns of cells grow down into the deeper layer carrying some of the 

 sex cells with them. Thus far (up to about the fifth week) the changes are 

 common to both sexes, and only later does the sex differentiation occur. 



After about the fifth week certain changes occur in the genital ridge which 

 differ accordingly as the ridge is to become an ovary or a testicle. In the case 

 of the ovary a layer of loose connective tissue grows in between the surface 

 epithelium and the cell columns or plugs mentioned above. The cell columns 

 are thus pushed farther from the surface and constitute the medullary cords. 

 These ultimately disappear. The surface epithelium again sends plugs of cells 

 (Pfl tiger's egg cords) down into the underlying tissue. These cords are made up 

 for the most part of epithelial cells which give rise to the follicular cells, but 

 contain also a considerable number of sex cells (primitive ova). The egg cords 

 then become broken up into smaller masses each of which contains a single 

 primitive ovum (rarely more) and constitutes a primitive Graafian follicle. 

 The sex cell grows in size and becomes the primary oocyte (and finally the mature 

 ovum), while the epithelial cells around it give rise to the stratum granulosum 

 and germ hill of the mature Graafian follicle. During these processes the stroma 

 also increases in amount, while the original germinal epithelium becomes reduced 

 to a single layer of cuboidal cells. The formation of egg cords is usually com- 

 pleted before birth. (See also p. 360.) 



In the case of the testicle a layer of dense connective tissue, the tunica 

 albuginea, develops between the germinal epithelium and the sex cords. The 

 epithelium becomes reduced to a single layer of flat cells. The sex cords which 

 first grew into the underlying tissue and which contain the sex cells, are destined 

 to give rise to the convoluted seminiferous tubules. This phase of development 

 dift'ers from that in the ovary inasmuch as in the latter case the first formed sex 

 cords (medullary cords) disappear, Plliiger's egg cords being formed later and 

 having no homologue in the testicle. The sex cords of the testicle become 

 more and more convoluted and the sex cells (spermatogonia) proliferate rapidly. 

 Beginning after birth and continuing up to the time of puberty, lumina appear 

 in the sex cords and they thus give rise to the convoluted seminiferous tubules. 

 The supporting cells (of Sertoli) are probably derived from the undifferentiated 

 epithelial cells of the sex cords. (See also p. 341.) 



