23 = 



UROGENITAL SYSTEM 



tissue and, with the spermatic vessels, constitute the spermatic cord. Owing 

 to the descent of the testis, the ductus deferens is looped over the ureter in the 

 abdomen (Fig. 251 C). In some cases the inguinal canals remain open so that the 

 testis may slip back into the abdominal cavity. Such conditions may lead to 

 inguinal hernia of the intestine. Open inguinal canals occur normally in the 

 rabbit. 



THE EXTERNAL GENITALIA 

 Indifferent Stage. — In both sexes there develops early in the midline of the 

 ventral body wall, between the tail and umbilical cord, the cloacal tubercle (Fig. 

 228). Upon this appears a knob-like structure, the phallus, and the two together 



Fig. 228. — Four stages in the development of the external genitalia in embryos of 24 to 34 mm. 

 Indifferent stage: 1, phallus; _•, glans; 3, primitive urogenital opening; 4, genital tubercle or swelling; 

 5, anus; 6, coccyx (Tourneux in Heisler's Embryology). 



constitute the genital eminence. Cranially about the phallus the cloacal tu- 

 bercle forms a crescent-shaped genital tubercle, which later gives rise to the right 

 and left genital swellings. The phallus grows rapidly and into it extends the 

 phallic portion of the urogenital sinus. At the end of the phallus the epithelium 

 of the sinus forms a solid urethral plate (Fig. 212). Along the anal surface of the 

 phallus in the midline, the wall of the urogenital sinus breaks through to the ex- 

 terior and forms the slitdike primitive urogenital aperture. In embryos of 21 to 

 28 mm., at the end of the phallus, the glans is marked off from the base by a cir- 

 cular groove, the coronary sulcus (Figs. 225 and 228 B). 



