340 THE INTERNAL ENVIRONMENT OF THE BODY Part TIT 



tivated by the entrance of the sperm cell. The puncture of an egg membrane 

 by a fine needle appears to arouse the egg as well as a puncture by a sperm 

 head. It seems that an egg may be as responsive to a physical starter as a 

 motor. 



Pedogenesis is parthenogenetic reproduction by a young, incompletely 

 developed animal. Its normal occurrence in a species is extremely rare. 



Neotcny. Under certain conditions tiger salamanders {Ambystoma ti- 

 grinuni ) that have not metamorphosed become sexually mature, mate, and 

 produce fertile eggs. This is neoteny, also a rare condition. 



Human Reproductive System 



Male 



In man, as in other mammals, the male reproductive system consists of a 

 pair of testes in which the sperm cells are produced and a series of ducts and 

 associated glands by which they are protected, nourished, and transported 

 (Fig. 18.5). 



Structure and Function. The testes lie in extensions of the body cavity 

 covered with skin, the scrotal sacs, that hang outside the body. Each testis 

 is the size of a walnut, about an inch long, smooth and oval. It consists of 

 hundreds of seminiferous or sperm-bearing tubules, each a foot or two 

 long, and the thickness of a coarse thread. All of them are tightly coiled in 

 an entanglement which requires exceedingly skillful dissection to unravel 

 (Fig. 18.6). Under the influence of a gonad-stimulating hormone of the 



URINARY 



BLAOOCR 



VAS DEFERENS 



PENIS 



BULBOURETHRA 

 GLAND 



TESTIS 



SCROTUM 



Fig. 18.5. Diagram of a section of human male reproductive organs showing 

 their relation to the urinary bladder and urethra. (Courtesy, Harbaugh and Good- 

 rich, eds.: Fundamentals of Biology. New York, The Blakiston Co., 1953.) 



