THE EXTERNAL GENITALIA 235 



descend into the vaginal sacs of the scrotum through the paired genital swellings, 

 as described on p. 231, but the scrotum itself is an unpaired structure derived 

 from the scrotal area. After the descent of the testes the genital swellings dis- 

 appear. 



Comparing the male and female external genitalia, it is plain that the glans 

 penis and glans clitoris are homologous. The labia minora correspond to the 

 phallic folds which close about the primitive urogenital opening and the anal 

 surface of the penis. The greater part of the stem of the male phallus does not 

 develop in the female. On the other hand, the genital swellings enlarge and be- 

 come the labia majora of the female, while in the male they are only temporary 

 structures. The scrotum does not develop in the female, being represented only 

 by the posterior commissure of the labia majora. 



The Prostate Gland. This is developed in both sexes as several outgrowths 

 above and below the entrance of the male ducts into the urogenital sinus. The tu- 

 bules arise in five distinct groups and, according to Lowsley (Amer. Jour. Anat., 

 vol. 13, pp. 299-350), number from 53 to 74, the average being 63. In the male 

 the surrounding mesenchyme differentiates both white fibrous connective tissue 

 and smooth muscle fibers into which the anlages of the prostate grow. In the 

 female the tubules remain isolated. The prostatic anlages appear hi male em- 

 bryos of 50 mm. (i2th week), chiefly as dorsal and lateral outgrowths. Two- 

 thirds of the tubules are caudal to the openings of the male ducts. In the female 

 the gland is rudimentary, the maximal number of outgrowths being three. 



The bulbo-urethral glands (of Cowper) arise in male embryos of 48 mm. as 

 solid paired epithelial buds from the entoderm of the pelvic urogenital sinus. 

 The glands grow into the mesenchyme which forms the corpus cavernosum urethra, 

 about which they enlarge. The glands branch and, at 120 mm., the epithelium 

 becomes glandular. The vestibular glands (of Bartholin) are the homologues in 

 the female of the bulbo-urethral glands. They appear in embryos of 36 mm., 

 grow until after puberty, and degenerate after the climacterium. 



Male and Female Genitalia Homologized. From the standpoint of embry- 

 ology the genital glands are homologous structures. In the indifferent stage 

 (Fig. 231 A), there are in both male and female a pair of genital glands, a pair of 

 mesonephric or male ducts, a pair of Muellerian or female ducts, and a genital 

 tubercle bearing the phallus. The genital ducts open into the urogenital sinus, a 

 part of which forms the bladder. 



Male (Fig. 231 C). In the male the Muellerian ducts degenerate except for 

 small portions cranially and caudally, which persist respectively as the appendix 



