Henry Gray (18251861). Anatomy of the Human Body. 1918.
(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).
FIG. 1118 Primitive kidney and bladder, from a reconstruction. (After Schreiner.) (See enlarged image)
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 urogenital 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. Skenes 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