as far as the oviduct openings in the four- 

 teenth segment; and the sperm (of the other 

 worm) from the receptacles pass into the 

 cocoon as soon as it reaches the ninth and 

 tenth segments. Finally, the worm wriggles 

 entirely free of the cocoon, slipping it off 

 over its tapered "head." Then the cocoon, 

 which now contains fertilized eggs, seals it- 

 self by drying and twisting at the ends. 



THE MALE REPRODUCTIVE TRACT OF 

 MAN 



In man and other vertebrates the repro- 

 ductive tract, especially in the male, develops 

 in close association with the excretory system. 

 Therefore the two systems together may be 

 called the urogenital system. This association 

 is mainly anatomical, however. Functionally 

 the two sets of organs are quite separate, ex- 

 cept that some of the ducts serve to transmit 

 both genital and excretory products. 



In vertebrates generally, the testes origin- 

 ate in the coelom just ventral to the em- 

 bryonic kidneys, and in lower vertebrates, 

 such as the frog, the adult gonads remain in 

 this position (Fig. 21-4). But in man and 

 many other mammals the testes descend 

 shortly before or after birth, taking up a 



Reproduction in Multicellular Animals - 383 



permanent position in the scrotum (Fig. 21-5). 

 The scrotum, essentially, is an outpocketing 

 of the body wall, which encloses a small por- 

 tion of the original coelom. However, the 

 inguinal canal, which initially is a free 

 channel connecting the scrotal and abdomi- 

 nal cavities, becomes blocked off by connec- 

 tive tissues. Thus in adults the scrotal and 

 abdominal cavities are separate, except in 

 cases of inguinal hernia. When herniation 

 occurs, the original channel reopens and in 

 severe cases a loop of the intestine may be 

 extruded into the scrotal sac. Rarely one or 

 both testes may fail to descend into the scro- 

 tum, resulting in unilateral or bilateral 

 cryptorchy. Moreover, such undescended tes- 

 tes fail to produce any functional sperm (p. 

 400). 



Primarily the testis is a mass of highly 

 coiled tubules, the seminiferous tubules (Fig. 

 21-6). These generate the sperm by a multi- 

 plication of the germinal epithelial cells lin- 

 ing the walls (Fig. 21-7). Secondarily, how- 

 ever, the testis is an endocrine gland. Packed 

 into the spaces between the tubules there is 

 a considerable mass of interstitial cells (Fig. 

 21-7). This interstitial tissue produces the 

 male sex hormone, testosterone, which is con- 

 cerned with the development of the second- 



Fig. 21-4. Diagrams of reproductive 

 organs of male ($) and female ($) 

 frog. Dotted lines show the connections 

 of the spermiducts with the ureter, 

 within the kidney. 



OVIDUCT 



U G CANAL 



SEMINAL 

 VESICLE 



CLOACA 



CLOACA 



cf 



9 



