DEVELOPMENT OF THE UBO-GENITAL OKGANS. 1333 



the bladder, and is derived from the ends of the Wolffian ducts (see above, p. 1332). 

 The remaining portion, beyond . the openings of the ductus deferentes of the adult, is 

 derived from the uro-genital canal, or caudal subdivision of the anterior part of the 

 cloaca. The uro-genital canal is early subdivided into a pelvic part lying within the 

 future pelvis minor and a penile part which occupies the region in which the corpus 

 cavernosum urethne is developed. The latter part of the uro-genital canal becomes 

 filled with closely and irregularly packed cells, which later, breaking down, re-establish 

 the canal and give origin to a slit-like opening in the region in front of the anus. The 

 canal for some time opens at a rhomboidal fossa situated in the groove at the base of the 

 glans. In the glans penis a septum of densely packed cells passes forwards from this 

 fossa and is known as the urethral septum. At a later stage these cells also break 

 down and form a groove, the lips of which unite and enclose the terminal portion of the 

 urethra. It is doubtful if any of the male urethra owes its origin to the ectoderm, but 

 there is some evidence to show that the urethral septum is to be regarded as ectodermal, 

 in which case the part of the canal which traverses the glans must have a like origin. 



THE FEMALE URETHRA. 



In the female the part of the urethra near the internal urethral orifice is developed from 

 the inferior ends of the Wolffian ducts and has an origin similar to that of the basal portion 

 of the bladder. The inferior part of the passage is derived from the uro-genital canal. When 

 the uro-genital canal opens on the surface it is continued forwards as a sulcus on the 

 genital eminence, as in the male sex. The margins of the slit-like opening do not unite, 

 but form the labia minora of the adult, and the sulcus which appears on the glans 

 3litoridis is closed without forming a canal. At first the fused caudal ends of the Mlillerian 

 lucts open into the uro-genital canal, but later a downgrowth, which is at first solid, 

 3stablishes a connexion between these ducts and the surface immediately in front of the 

 :-ectum and behind the opening of the uro-genital canal. This new connexion becomes the 

 vagina, and the uro-genital canal the urethra. By some embryologists it is believed 

 :hat a shortening and spreading out of the inferior portion of the uro-genital canal, to form 

 i part of the uro-genital cleft of the adult, is responsible for bringing the opening of the 

 'used Mullerian ducts to the surface. If this latter view is accepted, the female urethra 

 corresponds to the part of the male passage which lies above the opening of the utriculus 

 Drostaticus. 



THE SEXUAL GLANDS. 



Sexual Glands. In the development of the sexual glands, male and female, a 

 lifferentiated thickened portion of the peritoneal epithelium is first recognised. This 

 specialised epithelium, which has received the name of germinal epithelium, is situated to 

 hhe medial side of the mesonephros and of the Wolffian and Mullerian ducts. Here it 

 covers a longitudinally disposed ridge or elevation called the genital ridge. The germinal 

 epithelium is not strictly limited to this ridge, but extends to some extent beyond its 

 imits. The genital ridge is soon found to have numerous epithelial cells embedded in its 

 connective-tissue stroma which appear to originate, in both sexes, by a proliferation from 

 :he deep surface of the germinal epithelium covering the ridge. From these epithelial 

 cells the seminiferous tubules of the male, and vesicular follicles with their contained ova 

 )f the female are developed. The tissue which gives rise to the genital ridge occurs in 

 ill the body segments from the sixth thoracic to the second sacral, but the cephalic end 

 )f the ridge atrophies before the germinal epithelium can be recognised in the more caudal 

 segments, and only about one-fourth of the ridge gives origin to the permanent sexual 

 *land. The part of the genital ridge which persists appears to correspond to three or 

 'our segments in the region of the 4th or 5th lumbar to the 1st or 2nd sacral segments. 



In the male, as early as the thirty-third day, the epithelial cells embedded in the 

 itroma of the developing testis have become arranged into a network of anastomosing 

 cords within which certain larger cells are seen to be irregularly scattered. These larger 

 cells have received the name of primitive sperm cells, and are relatively few in number. 

 They undergo frequent division, and in the later stages are not to be distinguished 

 'rom the other cells of the cords. The cellular cords undergo direct transformation into 

 :he seminiferous tubules of the testis, the tubuli recti and the rete testis. At a very early 

 itage the superficial part of the stroma of the developing testis becomes denser, and gives 

 )rigin to the tunica albuginea. The tissue surrounding the cellular cords becomes 

 converted into the septula testis and the mediastinum. A lumen can first be recognised 

 n the seminiferous tubules in the seventh month. The rete testis becomes connected 



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