UROGENITAL SYSTEM. . 321 
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 Wolffian 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 
21 
