ment of the Kidney, and the Development of the - Journal of Cell ...

DEVELOPMENT OF AMPHIBIAN KIDNEY 449
actually the hinder end of the kidney at metamorphosis, the
true posterior region (Text-fig. 1, E and H) being added during
the succeeding years.
In the sexual region (Text-fig. 1, C and F) the elaboration of
the sexual connexion is the only change; this is described more
fully in a later part of this paper. No further functional units
are added, and the straight tubule maintains its primitive
appearance.
In the mid-region, however (Text-fig. 1, D and G), there is
a considerable increase in the number of functional units (FU.)
which become connected to the straight tubule (ST.). This last
loses its primitive straightness and becomes pulled out from one
side to another as more and more tubules become attached.
At the same time its histological appearance becomes less
sharply differentiated from the excretory tubules which surround
it. Thus there arises a condition when no further room
can be found for such attachments, and a number of collectingtrunks
(DC.) arise. These are produced in situ from the
blastema which always surrounds the inner angle of the kidney.
Units (DU.) also arise in this area, in the manner already
described (loc. cit.), and become attached to the collectingtrunks.
The whole course of the development of these units and
of the collecting-trunks themselves is highly irregular. Thus in
the middle region of a third-year kidney (Text-fig. 1, G) there
are some units regularly attached to the straight tubules and
others which communicate with the archinephric duct through
an irregular system of branched, and even sometimes anastomosing,
collecting-trunks. The abortive unit which terminated
the original straight tubule finally degenerates, and it becomes
very difficult to decide, even by reconstructional methods, what
was a straight tubule and what is a secondarily developed
collecting-trunk.
In the posterior region, which becomes differentiated during
the beginning of the second year, there are no straight tubules.
There are, however, a considerable number of functional units
which are connected solely to the collecting-trunk network.
These are shown developing in Text-fig. 1, E, and are diagrammatically
represented at the conclusion of their development