420 
RUTH RAND ATTERBURY 
The outer cells differ but slightly from the loose mesenchymal 
cells of the allantois. A number of them appear in the form of 
round, mobile cells. Their nuclei are large and vesicular with 
well-defined nucleoli. Their cytoplasm forms a narrow rim 
around the nucleus and appears to be more basophilic than that 
of the allantoic mesenchyme. The cell bodies are practically the 
same size as those of the surrounding mesenchyme. The cells 
in immediate proximity with the ureteric epithelium, on the other 
hand, have markedly hypertrophied—in some cases they appear 
two and three times the size of the more distal cells. The nuclei 
have become large and oval, containing prominent nucleoli; 
the cell bodies appear heavy and deeply basophilic. A difference 
in the metabolism of the two kinds of cells must have found its 
expression in this difference of structure. 
With the growth of the ureteric tubules of the second order, 
the continuity of the nephrogenous mantle is broken and the 
nephrogenous tissue becomes split into separate masses. Part 
is carried forward on the blind terminations of the secondary 
tubules; part remains behind and condenses to form compressed 
spherical cell masses in the angles between the primary and 
secondary ureteric branches and between the two secondary 
branches. Figure 4 shows the nephrogenous tissue split into four 
separate cell masses. In this way the continuous mantle of 
nephrogenous tissue, such as that illustrated in figure 3, becomes 
split into two caps of tissue covering the blind ends of the newly 
formed tubules and into a number of small islands, which remain 
in close relation with the primary ureteric branch. These islands 
of nephrogenous tissue are transformed into metanephric spheres 
similar to those of normal development. The close association 
between the metanephric spheres and the branches of the ureter 
bud is a constant one. In the grafts no metanephric sphere is 
found except in close relation with a ureteric tubule. The split¬ 
ting up of the nephrogenous tissue proceeds hand in hand with 
the division and subdivision of the ureteric branches. The 
process is not dissimilar from that usually described for the 
typical development within the embryo. 
