REMOVAL OF PRONEPHROS OF AMBLYSTOMA 
363 
3, a — a), lengthens rapidly in the region of the fourth myotome, 
and bending outward and downward in an acute angle over the 
upper surface of the yolk (fig. 4, h — h), forms a U-shaped loop. 
A little later this is bent over anteriorly, and may even come to 
lie slightly farther forward than the anterior nephrostome. 
The pronephros of Amblystoma differs from that of certain 
of the Anura in the possession of two instead of three nephros- 
tomal canals,^ and in the absence of the common chamber or 
'pronephric pouch,' the funnels, instead, narrowing directly 
C^ 
Figs. 5 and 6 Diagrams showing the region where greatest growth occurs in 
the early and late stages of development of the pronephros. Figure 5, condi- 
tion before the coiling of the longitudinal tubules connecting the funnels. Fig- 
ure 6, growth of these tubules in the older kidney, a.f., anterior funnel; ;)./., 
posterior funnel; c.t, longitudinal tubule; s.d. segmental duct; x-y, region of great- 
est growth during early development resulting in the formation of the ventro- 
lateral portion of the pronephric coil, p.c. Shaded areas drawn from wax models, 
pronephric coil indicated by curved lines. 
into the U-shaped tube just described. With further multi- 
plication of the cells just below the funnels, two longitudinal 
tubules are established (figs. 5 and 6, c.t., and fig. 17, L.T.), 
separating the anterior and posterior nephrostomes from each 
other and from their original point of junction with the looped 
tubule (x), as they grow. This growth is at first a very slow 
process as compared with that of the U-shaped portion. In the 
latter region the active proliferation of cells results in a rapid 
* One instance is on record of the presence of a third funnel on both sides in 
an Amblystoma larva. See Field ('91). 
