560 



VERTEBRATE LIFE AND ORGANIZATION 



Testis n 

 Nephron. — 

 Pe.ricaL-rdial 

 Cavity 



-Holonephros 



rlntestine 



''»//.,„.... 



-Opisthonephros 



'M////m 



^CloBcca. 



Pleuroperitonealca.vity-' "-Wolffia-n duct 



A B 



,,//,"r'n„ rWolffia.n duct 



•WolfFia.ixdu.ct 



Remnant oF 

 opisthonephroS 



Meta.nephros 

 ~- — Ureter 



Pronephros 



Perica.rdia.1 cavity 



D 



»^./^ Urinary ■ 

 bladder 



c 



Mesonephros 

 1% (developing) 



Gonei-d 



Wolffian, 

 duct 



Intestine. 



'^^Urinary 



blajlder 



Cloaca. 



'-Allantois 



Gonad 

 Oviduct- 



Bladder 



Urethra 



Genital tubercle- 



Pronephros 



(degenerating) 



MesoneplTros 



Wolffian 



duct 



■Metaneplii'os 

 (developing) 



P^^Ureter 



■Remnant of Mesonephros 



Wolffian duct 

 Metanephros 



Ureter 



■Rectum 



Cloac 



Figure 28.1. A comparison of the evolution and embryonic development of the 

 kidney and its ducts. A, B, C: the evolutionary sequence of kidneys. A, Hypothetical 

 ancestral vertebrate with a holonephros; B, a fish with an opisthonephros; C, a reptile 

 with a metanephros. D, E, The developmental sequence of kidneys in a reptile. F, A 

 mammalian embryo in which the cloaca is becoming divided by the growth of the 

 fold indicated by the arrow. The ventral part of the cloaca contributes to the urethra 

 in tlie male. It becomes further subdivided in the female and contributes to both 

 urethra and vagina. In both sexes, the dorsal part of the cloaca forms the rectum. 



In the kidney ot adult fishes and amphibians (Fig. 28.1 B), the 

 most anterior tubules have been lost, some of the middle tubules are 

 associated with the testis, and there is a concentration and multiplication 

 of tubules posteriorly. Such a kidney is known as a posterior kidney or 

 opisthonephros. 



Reptiles, birds and mammals (Fig. 28.1 C) have lost all of the mid- 

 dle tubules not associated with the testis, and have an even greater 

 multiplication and posterior concentration of tubules. The number of 

 nephrons is particularly large in birds and mammals; their high rate 

 of metabolism yields a large amount of wastes to be removed. It is 



