THE URINOGENITAL SYSTEM 391 



destined to become male or female. It has three divisions: 

 (1) the anterior or sexual division, containing the gonad, involves 

 about the anterior half of the Wolffian body; (2) a non-sexual 

 region of the Wolffian body occurs behind the gonad, and 

 (3) 'behind the Wolffian body itself the urinogenital ridge con- 

 tains only the Wolffian and Miillerian ducts. A transverse sec- 

 tion through the anterior division shows the following relations 

 (Fig. 221): on the median surface the gonad, on the lateral sur- 

 face near the dorsal angle of the body-cavity the Wolffian and 

 Miillerian ducts, the latter external and dorsal to the former: 

 between the gonad and ducts lie the tubules of the Wolffian 

 body destined to degenerate for the most part. 



There is an indifferent stage of the reproductive system 

 during which the sex of the embryo cannot be determined, either 

 by the structure of the gonad or the degree or mode of develop- 

 ment of the ducts. In those embryos that become males the 

 gonad develops into a testis, the Wolffian duct becomes the vas 

 deferens, the tubules of the anterior part of the Wolffian body 

 become the epididymis, those of the non-sexual part degenerate, 

 leaving a rudiment known as the paradidymis, and the Miillerian 

 duct becomes rudimentary or disappears. In embr3^os that be- 

 come females, the gonad develops into an ovary; the Wolffian duct 

 disappears or becomes rudimentary, the Miillerian duct develops 

 into the oviduct on the left side and disappears on the right side, 

 and the tubules of the Wolffian body degenerate, excepting that 

 functionless homologues of the epididymis and paradidymis per- 

 sist, known as the epodphoron and paroophoron respectively. 



It is not correct to state, as is sometimes done, that the 

 embryo is primitively hermaphrodite, for, though the ducts char- 

 acteristic of both sexes develop equally in all embryos, the primi- 

 tive gonad is, typically, only indifferent. Nevertheless, if the 

 gonad be physiologically as well as morphologically indifferent 

 in its primitive condition, the possibility of an hermaphrodite 

 development is given. The primitive embryonic conditions 

 appear to furnish a basis for any degree of development of the 

 organs of both sexes. 



Development of Ovary and Testis. Indifferent Period. The 

 reproductive cells of ovary and testis alike arise from a strip 

 of peritoneal epithelium, known as the germinal epithelium, 

 which is differentiated on the fourth day by its greater thickness 



