[17] The objection may be raised that a purely embryonic organ like the pronephros, which is functional in but few vertebrate adults and which originates in vertebrate embryos only to undergo atrophy, can have no other explanation than that of “recapitulation.” The objection, however, fails to take into account the possibility of the organ being serviceable to the _embryo_, in which it may be a provisory solution of the excretory problem and not a vestige of past ancestry. (2) _The mesonephric system_: This consists of a collection of nephridial tubules called the mesonephros (Wolffian body). The tubules of the mesonephros do not develop any duct of their own, but utilize the posterior portion of the pronephric duct, the said tubules becoming secondarily connected with this duct in a region posterior to the pronephridia (tubules of the pronephros). The pronephric tubules together with the anterior portion of the pronephric duct then atrophy, while the persisting posterior portion of this duct receives the name of mesonephric or Wolffian duct. The duct in question still terminates in the cloaca, and serves, in the male, the combined function of a urinary and spermatic duct; but, in the female, a special oviduct (the Müllerian duct) is superadded because of the large size of the eggs to be transmitted, the Wolffian or mesonephric duct subserving only the urinary function. The mesonephros is functional in mammalian embryos, but atrophies and disappears coincidently with the development of the permanent kidney. The same is true of amniotic vertebrates generally, except that in the case of reptiles the mesonephros persists for a few months after hatching in the adult, the definitive kidney of the adult being reinforced during that interval by the still functional mesonephros. In anamniotic vertebrates, however, no separation exists between the mesonephros and the metanephros, the two forming one continuous structure, the opisthonephros, which acts as the functional kidney of the adult. (3) _The metanephric system_: In the amniotic vertebrates the mesonephros and metanephros are distinct, the former being functional in embryos and in adult reptiles (for a few months after hatching), while the metanephros becomes the definitive kidney of the adult. The metanephros is a collection of nephridial tubules provided with a special urinary duct called the ureter, which empties into the bladder (not the cloaca). The Wolffian or mesonephric duct is retained as a sperm duct in the male (of amniotic vertebrates), but becomes vestigial in the female. Only a certain number of the nephridial tubules of the embryonic metanephros are taken over to form part of the permanent or adult kidney (in mammals, birds, and reptiles).