1144 UROGENITAL SYSTEM. 
is developed superficial to the cloacal membrane which separates the ectodermal cloacal 
fossa from the endodermal cloaca. This ectodermal part of the urethral canal is formed by 
the meeting of the lateral boundaries of the cloacal fossa, where they lie in front of the 
anal opening, and also by the closing of the lips of a groove which appears on the sexual 
eminence. The lateral boundaries of the fossa, fusing in the middle line, close in the 
portion of the canal which in the adult is Sirtonnded by the corpus spongiosum, while 
the portion which traverses the glans penis (7.e. the fossa navicularis) is formed from the 
groove which appears on the sexual eminence. The median raphe, extending from the 
anus forwards along the urethral surface of the penis in the adult, indicates ‘the line of 
fusion along which the spongy part of the urethra has been closed in and converted into a 
canal. The last portion of the fossa to be closed in is the extreme anterior part which 
lies just at the base of the glans. In some rare cases failure of the margins of the 
fossa to unite in this situation causes the urethra to appear to terminate at this 
point. In other cases a greater arrest of development leaves the cloacal fossa open, 
and we have a condition maintained which resembles to a certain extent the normal 
arrangement in the female. ‘To such abnormalities of the urethra the term hypospadias 
is applied. 
Female Urethra.—In the female the urethra is developed from the part of 
the anterior portion of the cloaca which lies below the developing bladder, and above 
the openings of the Millerian and Wolffian ducts. The part below these openings, 
the urogenital canal, during the course of development becomes shorter and_ shorter, 
and finally forms the epithelium at the bottom of the urogenital cleft immediately 
surrounding the urethral and vaginal openings. ‘Thus in the adult the urethra, which 
originally led into the urogenital canal, opens into the urogenital fissure. 
In some mammals, such as the hedgehog, the shortening of the urogenital canal does 
not take place in the female during development, and in these animals a single aperture 
exists for the urimary and genital systems. The hymen is placed where the fused 
Millerian ducts (vagina) open into the urogenital canal. 
Sexual Glands.—In the development of the sexual glands, male and female, 
a differentiated thickened portion of the peritoneal epithelium is first recognised. ‘This 
specialised epithelium, which has received the name of germinal epithelium, is situated to 
the inner side of the mesonephros and of the Wolffian and Miillerian ducts. Here it 
covers a longitudinally-disposed ridge or elevation called the genital ridge. The germinal 
epithelium is not strictly limited to this ridge, but extends to some extent beyond its 
limits. The genital ridge is soon found to have numerous epithelial cells embedded in 
its connective-tissue stroma which appear to originate, in both sexes, by a proliferation 
from the deep surface of the germinal epithelium covering the ridge. From these 
epithelial cells the seminal fables of the male, and Graafian follicles with their contained 
ova of the female are developed. 
In the male, as early as the thirty-third day, the epithelial cells embedded in the 
stroma of the deve loping testis have become arr anged into a network of anastomosing 
cords within which certain larger cells are seen to be ‘irregularly scattered. These larger 
cells have received the name of primitive sperm cells, and are relatively few in number, 
They undergo frequent division, and are in the later stages not to be distinguished 
from the other cells of the cords. The cellular cords undergo direct transformation into 
the seminal tubules of the testis, but the excretory apparatus, including the rete testis 
and tubuli recti, is derived by outgrowths from the anterior tubules of the mesonephros, 
At a very early stage the superficial part of the stroma, of the developing testis, becomes 
denser, and gives origin to the tunica albuginea. 
It has been stated that the sexual epithelium of the testis, unlike that of the ovary, 
is not derived from the germinal epithelium, but that it has its origin in an outgrowth of 
the mesonephros. The course of the development of the sexual glands is without doubt 
extremely difficult to follow. 
In the female large epithelial cells are found in the stroma of the developing ovary, 
beneath the germinal epithelium, as early as the thirty-third day. These primitive ova 
are much more numerous than the primitive sperm cells of the male, and form a very 
characteristic feature of the developing ovary. At first they lie isolated, but later—about 
the fifth week—they become surrounded by other smaller cells having a like origin from 
the germinal epithelium. Each primitive ovum surrounded by its cells becomes a 
primitive follicle, the further development of which has already been described (p. 1125). 
The proliferation of cells from the germinal epithelium goes on until the seventh month, 
and during the later stages the epithelium has the appearance of growing down into the 
stroma in the form of long branching cellular processes which break up into little nests of 
