THE UROGENITAL SYSTEM 403 



attested by their mesodermal origin and with the exception of the tubules 

 of the metanephros, their metameric arrangement. The pronephros in 

 vertebrates is an embryonic structure which persists only in the adults of 

 some cyclostomes, teleosts, and dipnoi. The mesonephros becomes the 

 functional kidney of adult anamnia, the metanephros of adult amniotes. 



Comparative embryology and anatomy suggest therefore, that the 

 function of excretion has phylogenetically migrated from the anterior 

 to the posterior part of the body. The possibility, however, cannot be 

 excluded, that originally every metamere in chordates, as in annelids, 

 contained a pair of excretory organs. But this assumption is weakened 

 by the evidence that the excretory tubules of acrania are not serially 

 homologous with those of craniotes. The possibility that the gonadic 

 sacs of amphioxus may be homologous with coelomoducts and kidney 

 tubules has, however, been suggested somewhat plausibly. 



PROTONEPHRIDIUM PRONEPHROS. MES0NEO"«0S METANEPHROS. 



PROTONEPHRICHA RENAL TUBULES GLOMERUU WOLFFIAN DUCT 



SECRETORY TUBULES. 



COLLECTING TUBULES' 



CHORDATE KIDNEYS. ureter' 



Fig. 332. — A diagram illustrating the four types of kidneys which occur in chordates. 

 The excretory tubules of amphioxus are ectodermal in origin like the protonephridia of 

 annelids, while they are mesodermal in vertebrates. In the course of phylogenesis the 

 excretory organs of chordates have migrated farther and farther back in the body, as is 

 shown in the diagram. 



Pronephros. The first vertebrate kidney to develop in ontogenesis, 

 and possibly also the oldest in phylogenesis, is the pronephros or head 

 kidney. This consists of three to fifteen pairs of segmental tubules, each 

 of which opens by a nephrostome into the body cavity. Each tubule is 

 connected laterally with the primitive pronephric duct which carries excre- 

 tions posteriorly to the cloaca. It is generally assumed that originally 

 each pronephric tubule, like the coelomoducts of annelids, had its own 

 aperture. The eHmination of liquid wastes by a common pronephric 

 duct must be considered as a secondary condition. The factors in this 

 phylogenetic development are obscure. Similar relations, however, are 

 found in some annelids. In some oligochaetes and hirudinea, a longi- 

 tudinal duct conveys the products of the testes to the outside. In others, 

 the nephridia may join a longitudinal duct which opens into the cloaca. 

 If, as is generally believed, the nephridia are ectodermal in origin, we 

 have here another case of convergence of structures which have a diverse 

 origin — the nephridia from the ectoderm, pronephros and duct from the 

 mesoderm. 



The pronephros functions in few adult vertebrates, but it appears to 

 function in the early ontogenesis of those craniotes which have little yolk 



