METANEPHRIC ANLAGE OF CHICK 
421 
The uriniferous tubules develop out of the separate meta- 
nephric spheres. Figure 5 represents a section through a graft 
of the metanephric anlage after three days’ growth in the allantois. 
Figure 6 represents another section through the same graft. 
Both photographs illustrate well the growth and differentiation 
ol the nephric tubules in grafts at early stages. In figure 5 the 
nephrogenous tissue is seen split up into numerous small cell 
masses, the metanephric spheres. In some of these the process 
of differentiation is well started. Each cell mass is closely as¬ 
sociated with a collecting tubule-—a branch of the ureter bud. 
Text figure C Camera-lucida drawing to illustrate formation of lumen. 
Metanephric graft of four days’ growth, c, capillary; M.S., metanephric sphere; 
Ms.c., mesenchymal cell; U.T., collecting tubule. 
This relationship is not clear in figure 5 for every cell mass, but 
the study of the graft in serial sections readily proves that each 
cell mass without exception is in close association with a ureteric 
tubule. In these preparations ureteric tubules are easily dis¬ 
tinguished from uriniferous tubules by their lighter stain. 
At first the cells forming the metanephric spheres bear no 
definite relation to each other, but are heaped together in a 
haphazard fashion; for example, as in the cell mass ( M.S .) in 
figure 4. During the second day’s growth in the allantois these 
cells begin to group themselves radially about a point near the 
