108 ANIMAL PHYSIOLOGY. 
nected with the duct, finally join it. This organ is continuous 
with the pronephros; in fact, all three (pronephros, mesone- 
phros, and metanephros) may be regarded as largely continua- 
tions one of another. 
The metanephros, or kidney proper, arises from mesoblast 
at the posterior part of the Wolffian body. The ureter origi- 
\ 
I, i 
he 
Fie. 126.—Section of the intermediate cell-mass of fourth day (Foster and Balfour, after Wal- 
deyer). 1x 160. m, mesentery ; L, somatopleure ; a’, portion of the germinal epithelium 
from the duct of Miiller is formed by involution ; a, thickened portion of the germinal 
epitbelium, in which the primitive ova C and o are lying ; £, modified mesoblast which 
will form the stroma of the ovary ; WK, Wolffian body ; y, Wolffian duct. 
nates first from the hinder portion of the Wolffian duct. In 
the fowl the kidney tubules bud out from the ureter as rounded 
elevations. The ureter loses its connection with the Wolffian 
duct and opens independently into the cloaca. 
The following account will apply especially to the higher 
vertebrates : 
The segmental (archinephric) duct is divided horizontally 
into a dorsal or Wolffian (mesonephric) duct and a ventral or 
Millerian duct. The Wolffian duct, as we have seen, develops 
into both ureter and kidney proper. 
To carry the subject somewhat further back, the epithelium 
lining the ccelom at one region becomes differentiated into col- 
umns or cells (germinal epithelium) which by involution into 
the underlying mesoblast forms a tubule extending from before 
backward and in close relation with the Wolffian duct, thus 
