Reproduction in Multicellular Animals - 385 



INTERSTITIAL CELLS 

 SPERMATOGONIA 



MATURE SPERM 

 SERTOLI CELL 



Fig. 21-7. Cross section of a sperm-forming 

 tubule of the human testis (semidiagrammatic). 



carry the sperm into the epididymis (Fig. 

 21-6), a part of the genital tract that is de- 

 rived from an embryonic kidney. The epi- 

 didymis in man has the appearance of a 

 compact mass that half encircles the testis 

 (Fig. 21-5), but actually the epididymis is a 

 very long (about 20 feet), much-coiled tube 

 that leads finally into the vas deferens, or 

 sperm duct proper. The vas deferens is a 

 relatively simple tube, with a thick muscular 

 wall, that leaves the scrotum and passes into 

 the abdominal cavity, through the inguinal 

 canal. In the abdominal cavity, the vas 

 deferens ends by joining the urethra, a short 

 stout tube that originates from the urinary 

 bladder. Thus the urethra fulfills a double 

 duty, in that it conveys both urine and semen 

 to the extremity of the penis (Fig. 21-5). 



In addition to the sperm tract proper, the 

 male reproductive system includes three pairs 

 of glands. A large seminal vesicle drains into 

 the vas deferens on each side, near the point 

 where the sperm duct joins the urethra (Fig. 

 21-5), and the prostate glands empty into the 

 urethra. In many mammals, the prostate is 

 plainly a double gland; but in man it ap- 

 pears as a single mass encompassing the root 

 of the urethra (Fig. 21-5). The prostate secre- 

 tion enters the urethra by two sets of fine 

 ducts that come in from either side of the 

 mass. And finally there are Cowper's glands, 

 a pair of glands that empty into the urethra 

 more distally (these small glands are not 

 shown in Figure 21-5). 



The composition of the semen is very com- 

 plex. In addition to the sperm, semen con- 



tains numerous glandular secretions, but the 

 precise origin of these secetions is somewhat 

 obscure. Probably some components of the 

 semen are derived from the epithelium of 

 the epididymis, and until the sperm have 

 passed through the epididymis they never 

 become motile. In fact the sperm do not gain 

 full motility until they make contact with the 

 secretions of the seminal vesicles; and the 

 sperm tend to be swept through the vas 

 deferens by peristaltic waves occurring in the 

 wall of this muscular duct. The sperm tend 

 to accumulate in the ampulla, a swollen part 

 of the vas deferens, near the point where it 

 receives the secretions of the seminal vesicle. 

 The vesicular secretion provides an ideal 

 medium for exciting and sustaining the swim- 

 ming activity of the sperm. This fluid is well 

 buffered with inorganic salts, and contains 

 an unusually high concentration of fructose, 

 which the sperm utilize in their metabolism. 

 However, the metabolism of the sperm ap- 

 pears to be largely anaerobic. Sperm decom- 

 pose the sugar only as far as lactic acid and 

 thus they utilize only a small fraction of the 

 potentially available energy. 



The secretions of the prostate and Cow- 

 per's glands are less clearly understood. 

 During sexual excitement these fluids appear 

 to precede the sperm through the urethra, 

 and possibly the secretions are purely lubri- 

 catory in function. 



As it traverses the penis, the urethra is 

 flanked on all sides by erectile tissue, dis- 

 posed in stout columns, called the corpora 

 cavernosa (Fig. 21-5). This spongy tissue is 



