276 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



c) The melanephroi: The third kidneys of vertebrates are the paired metanephroi. The 

 metanephroi arise in part from the remainder of the mesomere, posterior to the mesonephroi; 

 and in part as an outgrowth from the WoLffian ducts. This outgrowth occurs as an evagination 

 from the WoLffian duct near its connection with the cloaca; it pushes anteriorly and fuses 

 with that portion of the metanephros arising from the mesomere (Fig. 6oB and C). The 

 metanephros is thus an organ of double origin. That part which comes from the mesomere 

 develops into the tubules of the kidney and the renal corpuscles; that part derived from the 

 WoLffian duct becomes the collecting tubules and the spaces (calices) into which they empty. 

 The stalk of the outgrowth from the Wolffian duct becomes the metanephric duct or ureter, 1 

 which conveys the excretion of the metanephros to the cloaca. The structure of the meta- 

 nephros is similar to that of the mesonephros, except that nephrostomes are wanting. The 

 metanephric tubules are very long and complicated, with several loops and convolutions. The 

 metanephroi are the kidneys of the adults of all reptiles, birds, and mammals. 



From the foregoing account it will be seen that the three pairs of vertebrate kidneys arise 

 successively from the mesomere, each situated caudad to the preceding one. For this reason 

 it seems probable that they represent persisting parts of a kidney which originally extended 

 the entire length of the mesomere. The anterior part of this hypothetical kidney has gradually 

 degenerated so that the kidney appears to descend posteriorly. This disappearance of organs 

 from anterior segments is common in heteronomously segmented animals. 



3. The urinary bladder. The urinary bladder is in all forms above fishes a saclike evagina- 

 tion from the ventral wall of the cloaca. In fishes, the bladder when present is formed in part 

 by the enlargement of the terminal portions of the Wolffian ducts as well as by a cloacal evagina- 

 tion. In the embryos of reptiles, birds, and mammals, there is an enormous cloacal 

 evagination, the allantois, which may be regarded as a greatly expanded urinary bladder. 

 The allantois serves respiratory as well as urinary functions. The adult bladder develops at 

 the base of the allantoic stalk. Adult birds lack a urinary bladder. The excretory ducts 

 do not generally open into the urinary bladder directly, except in mammals. 



4. The gonads and their ducts. The gonads or reproductive organs consist of a pair of 

 lestes in the male, ovaries in the female. These organs arise as swellings, the genital ridges, 

 on the ventral wall of the mesomere, immediately to the medial side of the kidneys. They 

 project into the coelom and in the mature condition are generally provided with mesenteries. 



a) Male: The testis consists of a mass of tubules, the seminiferous tubules, which empty 

 into a network of tubules called the rete, which is situated usually along the attached part of 

 the testis. Tubular connections are established during embryonic stages between the rete 

 and the Bowman's capsules of the anterior part of the adjacent mesonephros (Fig. 6oC). 

 These connections are called the vasa efferentia, and they traverse the mesorchium into the 

 mesonephros. The vasa efferentia are then continuous within the mesonephros with certain 

 of the mesonephric tubules and through them find exit into the Wolffian duct. It thus happens 

 that in nearly all vertebrates the WoLffian duct serves as the male duct. In elasmobranchs and 

 Amphibia, in which the mesonephros is the functional adult kidney, the Wolffian duct in males 

 serves as both excretory and male duct (Fig. 62^4). In male reptiles, birds, and mammals, 

 where the mesonephros has disappeared, the WoLffian duct remains as the male duct, and is 

 then named the vas deferens (Fig. 62C). In these groups the mesonephros disappears, except 

 those mesonephric tubules which have established connections with the rete testis. These 

 tubules constitute the vasa efferentia and a part of the epididymis. Epididymis is the name 

 given to that part of the male duct which is convoluted. 



1 A great deal of confusion will be avoided by reserving the term ureter for the duct of the meta- 

 nephros. 



