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 Miullerian 
duct becomes rudimentary or disappears. In embryos that be- 
come females, the gonad develops into an ovary; the Wolffian duct 
disappears or becomes rudimentary, the Miullerian 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. Indijjerent 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 
