UROGENITAL SYSTEM. 32! 



into a seminiferous tubule, in the walls of which both the epithelial 

 cells and the primordial ova are recognizable, as well as a third kind 

 of cell, called Sertoli's cell, concerning which accounts are some- 

 what at variance, some regarding them as derivatives of the epithelial 

 cells, others as coming from the primitive germ cells. They play no 

 part in the actual formation of the spermatozoa, but act rather as 

 nutritive or 'nurse cells' for the developing spermatozoa. For the 

 differentiation of the germ cells into spermatozoa reference must be 

 made to the text-books of embryology and histology. In most verte- 

 brates the testes continue in the position where they first appear, but 

 in most mammals they eventually descend to a position outside of the 

 body cavity and are enclosed in a special pouch, the scrotum. This 

 descent of the testes is described in connection with the reproductive 

 organs of the mammals, below. 



THE REPRODUCTIVE DUCTS. 



The reproductive products formed in the gonads have to be carried 

 to the exterior, either as spermatozoa, or as eggs or young in different 

 stages of development, the ducts in the male being called vasa defer- 

 entia, those of the female being oviducts. The former are usually 

 the Wolman ducts, the latter may be either the Miillerian ducts or 

 tubes developed for the special purpose, or lastly, the abdominal pores. 



Male Ducts. In elasmobranchs, amphibia and amniotes the 

 Wolffian ducts (fig. 321) serve as the outlet for the sperm. While 

 the seminiferous tubules are developing, there occurs a proliferation 

 of cells from the wall of the Bowman's capsules in the anterior end 

 of the mesonephros. These medullary cords extend through the 

 adjacent connective tissue and into the genital ridge where they come 

 into close connexion with the developing seminiferous tubules (fig. 

 324). When the latter acquire their lumen the medullary cords also 

 become canalized, so that both form a continuous transverse tubule 

 (vas efferens) leading from the genital cells to the Malpighian cor- 

 puscles, and thence by the mesonephric tubules to the Wolffian duct 

 (fig. 325, A). These vasa efferentia become connected by a longi- 

 tudinal canal before entering the Wolffian body, while usually there 

 is another longitudinal canal connecting them in the body of the testis 

 (fig. 321, B). Usually this connexion of testis and Wolffian body 

 takes place at the anterior end of the mesonephros, but in some dipnoi 



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