510 
STRUCTURE OF VERTEBRATA. 
At a late period in those types in 
which the pronephros is a functional 
larval organ, but much earlier in the 
higher Vertebrates, another series 
of tubules is differentiated from the 
mesoblast, and, acquiring a con- 
nection’ with the segmental duct, 
constitutes the mesonephros, or mid- 
kidney. The tubules arise usually, 
though not invariably, nearer the 
posterior end of the body than the 
pronephros, and are formed from 
the portion of the mesoblast which 
connects the muscle segment and 
the lateral plate (see Fig. 270). 
Below the Amniota the mesonephros 
forms the permanent excretory 
organ. In higher forms another 
series of nephridial tubules arises 
still farther back in the body, and 
forms the metanephros, or perma- 
nent kidney. The mesonephric 
and metanephric tubules resemble 
each other closely, but the relation 
of the former to the pronephros 
is still a debated point. When 
fully developed, a mesonephric 
tubule consists of—(1) an internal 
ciliated funnel (nephrostome), which 
opens into the body cavity, but 
is only rarely represented; (2) a 
Fic. 270.—Development of excre- 
tory system of Vertebrate.—In 
part after Boveri. 
In I. the primitive segments are not 
separated off from the lateral plate, 
and the pronephros (/.) is seen arising 
from the lower part of the primitive 
segment. In II. the pronephros is com- 
pletely separated off from the primi- 
tive segment and lateral plate. In 
III. the origin of the mesonephric 
tubules is seen. They arise from the 
upper part of the lateral plate, which 
is now completely separated from the 
primitive segment, and curving round 
the pronephric duct come tg open into 
it. 
w.c., nerve cord; zch., notochord; Amn., 
pronephros; g., gut; /.s., primitive 
segment; es., mesonephric tubule; 
gn.d., pronephric duct; 4.c., body 
cavity; @o., aorta; szv., sub-intestinal 
vein, with vessel to the aorta. 
