THE URINOGENITAL 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.! 
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- 
1 [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. 
Liol. 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. ] 
