380 THE ABDOMEN AND PELVIS 



which lies caudal to the origin of the allantois (Fig. 89) is known as the 

 cloaca, and it becomes subdivided by a frontal (coronal) mesodermal septum 

 into ventral and dorsal portions. The ventral portion is differentiated into 

 a dilated upper part which forms the bladder, and a narrower lower part 

 which is termed the urogenital sinus, and the latter receives the openings of 

 the Wolffian ducts. The dorsal portion of the cloaca forms the rectum, and, 

 probably, other parts of the large intestine. 



Prior to this subdivision, the ectoderm and entoderm in the ventral wall 

 of the cloaca are in direct apposition no mesoderm intervening over an 

 area which is termed the doacal membrane. In the adult, this corresponds 

 roughly to the area between the symphysis pubis and the tip of the coccyx. 

 The septum which subdivides the cloaca reaches the cloacal membrane and 

 divides it into two parts, which are known as the urogenital and anal mem- 

 branes, respectively. About the eighth week of foetal life the urogenital 

 membrane breaks down, and the bladder thus acquires an opening on the 

 surface of the perineum. 



At the cephalic end of the cloacal membrane, the ectoderm becomes 

 heaped up to form an elevation, which, in the male, becomes the phallus. 

 Before the urogenital membrane breaks down, the urogenital sinus extends 

 forwards, so that after the rupture it opens not only on the perineum but 

 also on the caudal aspect of the phallus. As the latter rapidly increases in 

 the male to form the penis, the opening extends forwards and, pari passu, 

 becomes closed posteriorly, so that the urethra, which is thus derived from 

 entoderm, ultimately opens on the ventral surface of the penis at the base 

 of the glans. At a later stage, the urethra extends into the glans and the 

 orifice migrates forwards to the apex. It is still doubtful whether the terminal 

 part of the urethra is ectodermal or entodermal in origin. 



In the female, the genital eminence forms the clitoris, which is homologous 

 with the glans penis. 



Congenital Anomalies. (a) Hypospadias. The simplest variety of 

 hypospadias is due to persistence of the orifice at the base of the glans ; the 

 highest degree is found where the phallus remains diminutive, and the 

 membranous urethra opens on the surface of the perineum. Intermediate 

 degrees of this anomaly may occur. 



(b) In Epispadias, the urethra opens on the dorsal surface of the penis 

 near its attachment to the anterior abdominal wall. This abnormality is 

 due to rupture of the urogenital membrane at a point cephalic to the genital 

 eminence instead of caudal to it. 



(c) In some cases, the urogenital membrane is more extensive than 

 normal, and rupture cephalic to the genital eminence not only gives rise to 

 epispadias but the cleft is continued forwards and complete extroversion of the 

 bladder is produced. In this condition, the lower part of the anterior 

 abdominal wall is deficient and the two pubic bones fail to meet one another 

 in the middle line, so that the symphysis is absent. The posterior wall of the 

 bladder is exposed on the surface below the umbilicus and forms a somewhat 

 triangular red area, on which the ureters and genital ducts open. The 

 margins of the mucous membrane are continuous with the skin of the 

 abdominal wall. Complete extroversion of the bladder is always accompanied 

 by epispadias. 



The Scrotum. Two lateral swellings, which are termed the labio-scrotal 

 folds, lie one on each side of the genital eminence. As they grow tailwards. 

 they pass lateral to the perineal orifice of the urogenital sinus, and they blend 

 with a central swelling which occupies the middle line in front of the anus. 

 In the male, the scrotum is formed by this central swelling and the caudal 

 ends of the labio-scrotal folds. In the female, the labio-scrotal folds persist 

 in their entirety and constitute the labia majora. 



