THE ABDOMEN 871 



wards a transverse fold appears, which crosses the cloacal fossa in such 

 a position as to coincide with the posterior end of the internal partition. 

 This transverse fold is called the perineal fold. In the iemale it gives rise to 

 the perineum proper, and in the male it forms the tegmental parts of that 

 portion of the perineum which lies between the scrotum and the anus. 

 The part of the cloacal fossa behind the perineal fold forms the anal depres- 

 sion or proetodeam, and the portion of the cloacal membrane which forms it 

 is called the anal membrane. Wlien the anal membrane disappears (about 

 the third month) the anus becomes formed, and the rectal compartment of 

 the cloaca now opens into the back part of the cloacal fossa. The uro-genital 

 sinus, which is the posterior portion of the uro-genital compartment of 

 the cloaca, opens into the cloacal fossa in front of the perineal fold by 

 means of a narrow vertical cleft, called the uro-genital elelt. Shortly before 

 the completion of the division of the cloaca into its two compartments 

 (namely, about the fifth or sixth week), a prominence makes its appearance 

 at the anterior or cephalic part of the cloacal membrane. This prominence 

 receives the name of the genital or sexual eminence, and its importance con- 

 sists in the fact that the penis and cUtoris are developed from it. Leading 

 backwards from the genital eminence there is a furrow, which ends at the 

 uro-genital cleft. The genital eminence is surrounded at the front and sides 

 by an epiblastic fold, called the genital fold, which, as it extends backwards, 

 encloses between its two epiblastic layers the cloacal fossa and the cloacal 

 membrane. The posterior strrface of the genital eminence presents a groove, 

 which is continuous with the uro-genital furrow. The Ups of this groove, 

 by their subsequent meeting and fusion, form a canal, which represents the 

 part of the spongy urethra contained in the glans penis. Posterior to this 

 -the lips of the uro-genital furrow come together and unite as far back as the 

 uro-genital cleft, and thus convert the front part of the cloacal fossa into 

 a canal, which represents the spongy part of the urethra behind the glans. 

 The spongy part is thus to be regarded as a forward extension of the uro- 

 genit£d sinus, which opens at the uro-genital cleft. WTien the Ups of the 

 uro-genital furrow fail to meet and unite at any part, the condition known 

 as hypospadias is produced, in which the spongy urethra opens externally 

 on the under surface of the body of the penis. It is to be noted that, 

 whilst the prostatic and membranous portions of the urethra are developed 

 from the ventral or uro-genital compartment of the h\-poblastic cloaca, the 

 spongy portion is developed from the anterior part of tiie epiblastic cloacal 

 fossa, the posterior part of that fossa giving rise to the anus and anal canal. 



Structure of the VesiculaB Seminales. 



Each vesicula seminalis consists of a tube which is thrown into 

 a number of coils, these being held together by fibrous tissue. 

 WTien these coils are undone, and the tube straightened out, it 

 measiu'es from 5 to 6 inches in length. Its upper end is closed, 

 and along its course several diverticula are met w4th. Each 

 seminal vesicle in its natural sacculated condition is surrounded 

 by a sheath, which is derived from the recto-vesical lamina of the 

 Wsceral portion of the pelvic fascia. Within this sheath the tube 

 has a fibrous wall composed of delicate fibrous tissue, and within 

 this there is a muscular coat, which is formed of plain muscular 

 tissue arranged in two layers — outer longitudinal and inner circular. 

 Within the muscular coat is the mucous coat, which presencs a 

 number of reticular ridges with intervening alveoli. It thus bears 

 a resemblance to honeycomb, in which respect it corresponds with 

 the mucous membrane of the ampulla of the vas deferens and 

 of the gall-bladder, the last-named having coarser meshes. The 



