Chap. 18 



REPRODUCTION 



341 



Seminal duct- 

 -Tube drawn out 



TUbes coiled 

 in place 



Fig. 18.6. The human testis with a piece removed and some of the seminiferous 

 tubules drawn out of place. The sperm cells develop within the hundreds of these 

 threadlike seminiferous tubules. They mature as they pass through other ducts, 

 especially the epididymis, the much coiled single duct that lies along the side of 

 each testis. (Courtesy, Corner: The Hormones in Human Reproduction. Princeton, 

 N.J., Princeton University Press, 1942.) 



pars anterior of the pituitary gland, the sperms develop from cells in the walls 

 of the seminiferous tubules. They divide repeatedly, reduce their chromo- 

 somes to the half number, finally become very minute, and each develops 

 a single flagellum, a swimming tailpiece (Figs. 18.3 and 18,7). At first the 

 sperms cling to the supporting cells in the lining of the tubules, then they move 

 into the open channels, and are gradually carried toward the outer ducts. In 

 animals that breed the year round, such as rats, rabbits, and man, they are 

 produced more or less continuously. In those with limited annual breeding 

 seasons, such as birds, the production stops between seasons. 



The testes develop in the body cavity near the lower ends of the kidneys, 

 locations which the ovaries occupy throughout life. Before birth, however, 

 they gradually slip downward into the scrotal sacs. In man, this location is 

 permanent. In rats, rabbits, and several other mammals, the testes slip in and 

 out of the abdominal cavity. They are outside in the scrotal sacs during the 

 breeding season, and in the abdominal cavity between those seasons. If the 

 testes of certain animals abnormally remain in the body cavity, its higher tem- 

 perature destroys the sperm cells; such testes are said to be hidden or crypt- 



