268 STRUCTURE OF THE VERTEBRATES 



comes in contact with the scrotal wall and develops the mus- 

 cular lining of the sac. Each vas deferens passes from the 

 epididymis over the pubic bone, through the inguinal canal, and 

 around the urinary bladder to its entrance into the urethra. 



The semen develops as a fluid medium for the sperms. These 

 cells are motile and live best in an alkaline medium. Part of 

 the fluid is secreted by the tubules of the testis and epididymis, 

 but the major portion is secreted by several sets of glands 

 located around the urethra. The prostatic fluid is strongly alka- 

 line and neutralizes the urethral passage. Near the prostate are 

 the paired seminal vesicles, and more distally, the small Cow- 

 per's glands. Small lubricating glands, which have no connection 

 with the seminal fluid, are found near the glans. 



D. Differentiation of Sex in the Human 



Except for the fundamental difference in sex as determined 

 by the chromosomes at fertilization, much of sex differentiation 

 is controlled by the secretion of the ductless glands. Sex char- 

 acters are divided into the primary characters, those structures 

 wliich are directly related to the reproductive function; and the 

 secondary characters, which develop later and are simply mani- 

 festations of sex difference. The growth or degeneration of the 

 different internal reproductive structures has been discussed. 

 The following is limited to the external organs. 



In the early, undifferentiated, human embr^^o the cloaca is 

 present and the external orifice is bounded by nearly circular 

 folds. Near the ventral margin of the fold appears a genital 

 tubercle resembling a small papilla. Synchronously with the 

 growth of this tubercle the anal opening is separated from the 

 urinogenital region. The tubercle grows larger and the lateral 

 folds become more prominent. At this stage the sexes are still 

 externally undifferentiated. 



From this time on the sexes rapidly take on their typical 

 characteristics. In the male the tubercle enlarges and is joined by 

 two small inner folds. The latter develop into the erectile or 

 cavernous bodies of the penis. The tubercle forms the urethral 

 body and the glans. The larger, outer, genital folds push ven- 

 trally, unite, and form the scrotum into which the testes descend. 



