THE ABDOMEN 



6S5 



form the columns of Sertoli. (2) The spermatogonia undergo mitotic division, 

 and give rise to a second layer of cells, called the primary spermatocytes, or 

 mother-cells. (3) The primary spermatocytes also undergo mitotic division, 

 and give rise to a third layer of cells, 

 called the secondary spermatocytes, or 

 daughter-cells. (4) The secondary sper- 

 matocytes also undergo mitotic division, 

 and give rise to a fourth layer of cells, 

 called the spermatoblasts, or spermatids. 

 These spermatids, having undergone 

 considerable modifications, give rise to 

 the spermatozoa. These spermatozoa lie 

 with their heads buried between the more 

 deeply placed cells, and their long tails 

 projecting free into the lumen of the 

 tubule. The enlarged ends or heads of 

 the spermatozoa, whilst they lie buried 

 between the deeper cells, are connected 

 with the sustentacular cells of the lining 

 epithelium. The tubuli recti are composed 

 of a basement membrane lined with a 

 single layer of cubical epithelium. The 

 tubules of the rete testis are destitute of 

 a basement membrane, its place being 

 taken by the connective tissue of the 

 mediastinum. The lining membrane of 



the tubules consists of a single layer of cubical cells. The vasa eflerentia and 

 coni vasculosi are composed of a basement membrane, external to which there 

 is a layer of plain muscular fibres arranged in a circular manner. The lining 

 epithelium is of the ciliated columnar variety. 



The structure of the epididymis is similar to that of the vasa eflerentia 

 and coni vasculosi. 



Fig. 297. — The Tubuli Semini 

 lERi OF THE Testis (magni- 

 fied). 



Development of the Internal Sexual Organs. 



The testis and the ovary do not present any distinctive sexuzd characters 

 in the earliest stage of their development, and this stage, in each case, is 

 therefore spoken of as the indifferent stage. 



Indifferent Stage. — The first indication of a sexual organ is the appearance 

 of a thick longitudinal fold on the inner surface of the Wolffian body. This 

 fold is called the genital ridge, and it is formed by (i) a thickening of the 

 coelomic epithelium which covers the inner surface of the Wolffian body, 

 and (2) the subjacent mesodermic tissue. 



The genital ridge presents three regions — namely, anterior or rete region. 

 middle or sex-gland region, and posterior or mesenteric region, the last-named 

 taking no part in the development of the sexual organ. 



The ccelomic epithelium of the genital ridge constitutes the germinal 

 epithelium, and the cells of this epithelium over the sex-gland region of the 

 genital ridge invade the subjacent mesoderm in the form of strands called 

 the sex-cords. The cells of these strands are of two kinds, some being small 

 epithelial cells, whilst others are large primitive sexual cells. As the stage of 

 differentiation proceeds, these sexual cells give rise to spermatozoa, or seminal 

 cells, in the case of the testis, and to ova in the case of the ovary. 



The indifferent sexual organ is thus represented by the genital ridge, the 

 mesodermic basis of which is invaded by sex-Cords, the cells of which are 

 partly epithelial and partly sexual. Moreover, the indifferent organ hes on 

 the inner side of the Wolffian body. 



The stage of differentiation now commences. 



