THE UKINOGENITAL SYSTEM 

 THE PRONEPHROS AND THE PRIMITIVE KIDNEY 



IN all classes of Vertebrates the Urinogenital System first appears 

 [in the form of a duct (Wolffian or Segmental duct) which is 

 primarily related to a urinary apparatus confined to the head 

 region. In the Amniota and Selachii the latter is wholly de- 

 generate in character ; among the remaining Anamnia, however, 

 it may for a longer or shorter period persist as a distinct first- 

 formed functional excretory organ. It is accordingly regarded 

 as a possible larval kidney, and termed the pronephros, as it 

 appears to be of very ancient origin]. While the secreting 

 glandular portion of this system never lasts for more than a 

 short period, its duct persists and appears in some cases (cf. 

 infra, p. 190) to give rise to the leading duct of a much more 

 extensive urinary system that develops later and is known as the 

 middle kidney or mesonephros. 



This second nephridial system, which becomes the definitive 

 urinary system of Fishes and Amphibia, consists like the pro- 

 nephros of metamerically recurrent tubes. The two systems 

 are so constituted as to suggest for the Vertebrata of to-day an 

 origin from a lowly segmented ancestor. 1 



The higher Vertebrates pass through an embryonic stage, in 

 which they possess first a pronephros and then a mesonephros, 

 which is an irrefragable proof that in their ancestors, and con- 



3 [This view receives support from the general tendency towards corresponding 

 metamerism of the muscular, skeletal, nervous, and vascular systems of the vertebrate 

 body. There are, however, reasons for thinking that the recurrent symmetry of at least 

 the skeletal and muscular apparatus may be of secondary significance ; and there are 

 not wanting competent investigators who deny in toto the origin of Vertebrates from 

 multi-segmented animals (cf. especially W. K. Brooks "The Genus Salpa, " Mem. 

 Biol. Lab., Johns Hopkins Univ., II. pp. 182-203). The whole question must remain 

 in abeyance, pending further inquiry into the origin of metamerism in general, with 

 a view to the formation of a sounder conception concerning that.] 



