DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS 1213 



(3) a posterior phallic portion, closed externally by the urogenital membrane (Fig. 

 1118). The second and third parts together constitute the urogenital sinus. The 

 vesico-urethral portion absorbs the ends of the Wolffian ducts and the associated 

 ends of the renal diverticula, and these give rise to the trigone of the bladder and 

 part of the prostatic urethra. The remainder of the vesico-urethral portion forms 

 the body of the bladder and part of the prostatic urethra; its apex is prolonged to 

 the umbilicus as a narrow canal, which later is obliterated and becomes the medial 

 umbilical ligament (urachus). 



Wolffian duct - 

 Hind-gut 



Outer zone\ 



> of kidney 

 Inner zone) 



Bladder 



Pelvis of kidney 



Urogenital 

 membrane 



FIG. 1118. Primitive kidney and bladder, from a reconstruction. (After Schreiner.) 



The Prostate. The prostate originally consists of two separate portions, each 

 of which arises as a series of diverticular buds from the epithelial lining of the uro- 

 genital sinus and vesico-urethral part of the cloaca, between the third and fourth 

 months. These buds become tubular, and form the glandular substance of the two 

 lobes, which ultimately meet and fuse behind the urethra and also extend on to its 

 ventral aspect. The isthmus or middle lobe is formed as an extension of the lateral 

 lobes between the common ejaculatory ducts and the bladder. Skene's ducts in the 

 female urethra are regarded as the homologues of the prostatic glands. 



The bulbo-urethral glands of Cowper in the male, and greater vestibular glands 

 of Bartholin in the female, also arise as diverticula from the epithelial lining of the 

 urogenital sinus. 



The External Organs of Generation (Fig. 1119). As already stated (page 1109), 

 the cloacal membrane, composed of ectoderm and entoderm, originally reaches from 

 the umbilicus to the tail. The mesoderm extends to the midventral line for some 

 distance behind the umbilicus, and forms the lower part of the abdominal wall; 

 it ends below in a prominent swelling, the cloacal tubercle. Behind this tubercle 

 the urogenital part of the cloacal membrane separates the ingrowing sheets of 

 mesoderm. 



The first rudiment of the penis (or clitoris) is a structure termed the phallus; it 

 is derived from the phallic portion of the cloaca which has extended on to the 

 end and sides of the under surface of the cloacal tubercle. The terminal part of 

 the phallus representing the future glans becomes solid; the remainder, which 

 is hollow, is converted into a longitudinal groove by the absorption of the 

 urogenital membrane. 



In the female a deep groove forms around the phallus and separates it from the 

 rest of the cloacal tubercle, which is now termed the genital tubercle. The sides of 



