THE EXTERNAL GENITALIA 



225 



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. 238 C). 



In the female, shallow peritoneal pockets, frequently persistent as the 

 diverticula of Nuck, correspond to the vaginal sacs of the male. Rarely 

 a more or less complete descent of the ovary into the labium majus occurs. 



Anomalies. At times, the testes remain undescended in the abdomen, a condition 

 known as cryptorchism and associated with sterility in man. In some mammals (whale, 

 elephant) it is the normal condition. The inguinal canals of man may remain open and 

 allow the testes to slip back into the abdominal cavity. Such conditions lead to inguinal 

 hernia of the intestine. Open inguinal canals, with a periodic descent during the breeding 

 season, occur normally in some animals (rodents, bats). 



C. THE EXTERNAL GENITALIA 



Indifferent Stage. The external genitalia of both sexes are similar 

 until the beginning of the third month of development, when the indifferent 

 anlages become moulded into sexually distinct organs. There develops 

 early in the midline of the ventral body wall, between the tail and um- 

 bilical cord, the cloacal tubercle. Upon this appears a knob-like struc- 

 ture, the phallus, and the two together constitute the genital eminence (Fig. 

 220). The cloacal tubercle forms about the base of the phallus genital 

 swellings, more pronounced laterally. The phallus grows rapidly, carry- 

 ing with it the phallic portion of the urogenital sinus (Fig. 219). At the 

 end of the phallus the epithelium of the sinus forms a solid urethral plate. 



A B C 



FIG. 235. Three stages in the developement of the external genitalia in human embryos of 

 24 to 34 mm. (after Tourneux in Heisler). Indifferent stage: I, Phallus; 2, glans; 3, primitive 

 urogenital opening; 4, genital tubercle or swelling; 5, anus; 6, coccyx. 



Along the anal surfaces of the phallus, in the midline, the wall of the uro- 

 genital sinus breaks through to the exterior and forms the slit-like, prim- 

 itive urogenital opening (Fig. 235). In embryos of 21 to 26 mm., at the 

 end of the phallus, the glans is marked off from the base by a circular 

 groove, the coronary sulcus (Figs. 232 and 235.6). 



Female. A deep groove appears about the base of the phallus, separa- 

 ting it from the genital swellings, which become circular (Fig. 235 C). 

 From the swelling differentiates : (i) cranially, the mons pubis; (2) laterally, 



