276 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



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

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

 and in part as an outgrowth from the Wolffian ducts. This outgrowth occurs as an evagina 

 from the Wolffian duct near its connection with the cloaca; it pushes anteriorly and fi 

 with that portion of the metanephros arising from the mesomere (Fig. 60B and Q. 

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

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

 Wolffian duct becomes the collecting tubules and the spaces (calices) into which they em] 

 The stalk of the outgrowth from the Wolffian duct becomes the melanephric duct or ure 

 which conveys the excretion of the metanephros to the cloaca. The structure of the mi 

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

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

 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 a 

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

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

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

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

 from anterior segments is common in heteronomously segmented animals. 



3. The urinary bladder. — The urinary bladder is in all forms above fishes a saclike evagi 

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

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

 tion. In the embryos of reptiles, birds, and mammals, the urinary bladder is enormoi 

 enlarged and extends out beyond the limits of the body, becoming the respiratory organ 

 allantois, of the embryos. The adult bladder is but the small proximal portion of the allant 

 the remainder vanishing. In birds the urinary bladder is present in the embryonic stage oi 

 The excretory ducts do not generally open into the urinary bladder directly, except in mamm 



4. The gonads and their ducts. — The gonads or reproductive organs consist of a pai: 

 testes in the male, ovaries in the female. These organs arise as swellings, the genital rid 

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

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



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

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

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

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

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

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

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

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

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

 serves as both excretory and male duct (Fig. 62.4). In male reptiles, birds, and mamm 

 where the mesonephros has disappeared, the Wolffian duct remains as the male duct, an 

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

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

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

 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 m 

 nephros. 



