METANEPHRIC ANLAGE OF CHICK 
423 
4 he vesicles continue to grow and shortly acquire a triple 
bend forming the S-shaped structure so characteristic of the 
normally developing metanephros. Soon the blind end of the 
S-shaped tubule is seen in contact with the adjacent ureteric 
tubule. An opening is then established, the cells of the separat¬ 
ing membrane being pushed aside and rearranged. In this way 
the nephric epithelium becomes continuous with the ureteric 
epithelium. From this time on growth of the tubule proceeds 
by mitotic division of the epithelial cells. 
Figure 5 shows an S-shaped tubule (*S). Its structure differs 
in no respect from the S-shaped tubule of the normal metanephros. 
The section following that from which figure 5 has been photo¬ 
graphed shows that this tubule has already acquired an opening 
into the collecting tubule (l .T.). Just as in the normally develop¬ 
ing uriniferous tubule, the S-shaped tubule of the grafts may be 
divided into three parts, an upper limb, middle piece, and lower 
limb. The distal part of the lower limb gives rise to Bowman’s 
capsule. In the S-shaped tubule (S) of figure 5, the cells of 
the outer wall of the lower limb already appear flattened. The 
S-shaped tubule (S) of figure 6 is in a still more advanced stage 
of differentiation. The distal end of the lower limb has grown 
around the structure in the form of a crescent, enclosing the 
upper limb within its arc. The cells of the outer or parietal layer 
are simple, flattened, and polygonal. The cells of the inner 
or visceral layer are likewise simple, but large and columnar 
and of a peculiar elongated pear-shape. The narrow interval 
between the two layers constitutes the uriniferous chamber. 
The same figure shows a Bowman’s capsule ( B.C .) in a still 
later stage of differentiation. The capsule has increased in 
size, its crescentic form tending to become more spherical. The 
urinary chamber appears greatly distended-—a fact which might 
be taken as indirect evidence for the functional activity of the 
cells at this time. 
The process of differentiation of the metanephric anlage in 
the allantois proceeds still further, leading to the appearance of 
typical glomeruli—structures which were not present at the 
time of grafting and which indeed do not appear in the normal 
