THE UROGENITAL SYSTEM — EXCRETION AND REPRODUCTION 551 



estimated that man has 1,000,000 or more nephrons per kidney whereas 

 certain salamanders have less than 100. The tubules concerned with 

 urine production drain into a ureter, which evolved as an outgrowth 

 from the Wolffian duct. The W^olffian duct itself has been taken over 

 completely by the male genital system. The kidney of the higher verte- 

 brates is known as a metanephros. 



The evolutionary sequence of kidneys is holonephros, opistho- 

 nephros and metanephros. In the development of vertebrates, we find 

 a slightly different sequence, but one that also involves a posterior 

 concentration of kidney functions (Fig. 28.1 D, E). In an early embryo 

 of a reptile, for example, segmentally arranged tubules appear dorsal to 

 the anterior end of the coelom, form the Wolffian duct, and disappear. 

 Ihese transitory tubules constitute a pronephros. Then a middle 

 group of tubules, known as the mesonephros, appear and connect with 

 the Wolffian duct (Fig. 28.3). These function during much of embryonic 

 life, but when the metanephric tubules develop, all of the mesonephric 

 tubules are lost except for those associated with the testes. The em- 

 bryonic sequence of kidneys in the development of a higher vertebrate 

 is pronephros, mesonephros and metanephros. 



A urinary bladder, for the temporary accumulation of urine, is 

 associated with the excretory ducts of many vertebrates. Most tetrapods 

 have a bladder, which develops as a ventral outgrowth from the cloaca. 

 Generally the excretory ducts from the kidneys lead to the dorsal part 

 of the cloaca, and urine must flow across it to enter the bladder, but in 

 mammals (Fig. 28.1 F) the ureters lead directly to the bladder, and the 

 bladder opens to the body surface through a short tube, the urethra. 

 The cloaca becomes divided and disappears as such in all but the most 

 primitive mammals. The dorsal part of the cloaca forms the rectum 

 and the ventral part contributes to the urethra of higher mammals 

 (Fig. 28.1 F). 



Urine is produced continually by the kidneys, and is carried down 

 the ureters by peristaltic contractions. It accumulates in the bladder, 

 for a smooth muscle sphincter at the entrance of the urethra and a 

 striated muscle sphincter located more distally along the urethra are 

 closed. Uruie is prevented from backing up into the ureters by valvelike 

 folds of mucous membrane within the bladder. When the bladder 

 becomes filled, stretch receptors are stimulated and a reflex is initiated 

 which leads to the contraction of the smooth muscles in the bladder 

 wall and the relaxation of the smooth muscle sphincter. Relaxation of 

 the striated muscle sphincter is a voluntary act. 



239. The Nephron and Its Function 



Nephron Structure. The excretory ducts and the urinary bladder 

 are important adjuncts to the kidneys, but the essential work of the 

 system, the selective removal of materials from the blood, is performed 

 by the individual kidney tubules. The general nature and function of 

 these tubules was described in Chapter 5. The mammalian nephron 

 may be taken as an example. The proximal end of each nephron (Fig. 



