8/0 A MANUAL OF ANATOMY 



is returned by the dorsal vein of the penis, and by the internal 

 pudic venae comites. 



For the structure of the different parts of the urethra, see Index. 



The glans penis and corpora cavernosa are developed from the genital emi- 

 nence and the corpus spongiosum is developed from the internal genital folds. 



Development of the Bladder and Urethra — The AUantois. — The allantois 

 is a hypoblastic diverticulum from the ventral wall of the hind-gut, which, 

 with an investment of mesoblast, extends into the space between the true 

 and false amnion. There are, therefore, two constituent parts in the 

 allantois, namely, the hypoblastic diverticulum and its mesoblastic invest- 

 ment. This latter receives the embryonic allantoic or umbilical arteries, 

 and, being very vascular, it carries bloodvessels to the villi of the chorion. 

 The allantois thus establishes a communication between the hind-gut, which 

 is intra-embryonic, and the chorion, which is extra-embryonic, and function- 

 ally it is associated with the nutritive and respiratory processes of embry- 

 onic life. The extra -embr^'onic and intia embryonic parts are in continuity 

 with each othei through the umbilical apening. The former is of interest 

 on account of its mesoblastic constituent, which, as has been stated, carries 

 blood to the chorionic viUi, and gives ase to the foetal element of the 

 placenta. The intra-embryonic part ol the allantois is at first directly con- 

 tinuous with the upper part of the bladder, but in the course of the second 

 month its lumen usually disappears, and then this portion is transloimed into 

 a fibrous cord, which is called the urachus. It is, howe\er, to be noted that 

 the lumen of the intra-embryonic part of the allantois may remain per- 

 sistent for some time, thus giving rise to the condition which is known, 

 after birth, as an umbilical urinary fistula. 



The cloaca, which is the common blind chamber of the' hind-gut and 

 uro-genital ducts, becomes divided into two compartments — dorsal and ventral 

 — by the cloacal or uro-rectal septum. The dorsal compartment gives rise to 

 the rectum and upper two-thirds of the anal canal, or portion above the anal 

 valves, whilst the ventral compartment constitutes the uro-genital sinus. 



The proximal end of the allantois is connected with the cephalic end of the 

 uro-genital sinus, and the two Wolffian ducts, together with the two Miillerian 

 ducts (now fused), open into it. Further, the ureter of either side is a diverti- 

 culum of the corresponding Wolffian duct close to the uro-genital sinus. 



The urinary bladder is developed from the cephalic part of the uro-genital 

 sinus. As the vesical dilatation proceeds, the ureters lose their connection 

 with the Wolffian ducts, and are transferred to the dorsal portion of the 

 developing bladder, into which they now open. The terminal portions of the 

 Wolffian ducts, being shifted downwards, give rise to the common ejaculatory 

 ducts, which open into the caudal part of the uro-genital sinus. 



Prostatic Urethra. — The upper portion of the prostatic urethra, as low as 

 the sinus pocularis, is, like the bladder, developed from the cephalic part of 

 the uro-genital sinus, and in the female the upper part of the urethra is formed 

 from it. The lower portion of the prostatic urethra is developed from the caudal 

 part of the uro-genital sinus, as is also the lower part of the female urethra. 



Membranous Urethra. — The membranous portion of the urethra is developed 

 from the caudal part of the uro-genital sinus. 



Development of the Spongy Portion of the Urethra. — The cloaca, wliich is 

 the common opening of the hind-gut and uro-genital ducts, is bounded behind 

 and below by a membrane called the cloacal membrane. This membrane in 

 very early embryonic life shuts off the cavity of the hind-gut from the exterior. 

 It is composed of two layers, internal or hypoblastic belonging to the hind-gut, 

 and external or epiblastic, which is an invagination of the epiblast. The 

 cloacal membrane forms the floor of a fossa, called the cloacal fossa. In the 

 course of the second month the cloaca itself becomes partitioned off into two 

 compartments by a septum formed of two lateral transverse portions, which 

 fuse in the middle line. The dorsal division is called the rectal compartment, 

 and the ventral is termed the uro-genital compartment. Shortly after- 



