422 
RUTH RAND ATTERBURY 
center of the metanephric sphere, the cells nearest the center 
acquiring a distinct epithelial arrangement. This stage is well 
illustrated in figure 5 ( M.S .), and in figure 6 (. M.S .). The central 
cells resemble the proximal, epithelially arranged cells of the 
nephrogenous mantle described in figure 3. They appear larger 
than the more peripheral cells of the cell mass with large oval 
nuclei. During this phase of development the cells are deeply 
basophilic. Soon, however, the central, radially arranged, 
epithelial-like cells undergo a change. At their pointed central 
ends the cytoplasm begins to take a faintly pinkish stain—a 
condition which might be considered a precursor of secretory 
activity. The central pointed ends are then flattened and pushed 
apart by the appearance of an acid-staining fluid at the point 
upon which the radially arranged cells had previously converged. 
Text figure C is a camera-lucida drawing illustrating this phenom¬ 
enon. A distinct lumen is formed in later stages. It seems, 
therefore, that the formation of a lumen may be due to the prod¬ 
ucts of secretory activity of the epithelial-like cells, which 
force the appearance of a space in the center of the metanephric 
sphere and subsequently its gradual widening. An analogous 
process was observed during the formation of the lumen in the 
normally developing metanephros of a nine-day chick embryo. 
In this manner the metanephric sphere is converted into a 
thick-walled metanephric vesicle. A metanephric vesicle is 
found in figure 6 (M.V.). The vesicle grows, its lumen widens, 
and the whole structure appears to develop distally from the 
collecting tubules with which it is associated. The growth of the 
vesicle takes place not so much through mitotic division of its 
cells, as by addition of cells from the more peripheral parts of 
the sphere. Mitotic figures at this stage of development are 
not as numerous as one would expect. The outermost cells of 
the sphere, too, eventually seem to contribute to the formation 
of the nephric tubules. They gradually become exhausted, 
only a few cells remaining behind in the form of scattered mesen¬ 
chymal cells situated among the developing renal tubules. The 
sharp distinction of the nephrogenous tissue into inner and 
outer zones, therefore, does not seem to be supported by these 
observations. 
