METANEPHRIC ANLAGE OF CHICK 
417 
part of the anlage, which, at the time of grafting, is slightly less 
advanced in the process of differentiation. Just as in typical 
development, the primary ureteric branches grow in the allantois 
and carry with them parts of the nephrogenous tissue which 
closely cap their somewhat expanded ends. After the branches 
have attained a certain length, dichotomous division takes place 
at their blind terminations, giving rise to secondary tubules. 
This usually occurs at the end of the first or in the early part of 
the second day of growth in the allantois. Figure 3 represents 
a section through a graft of the metanephric anlage after twenty- 
four hours’ growth in the allantois and illustrates well the branch¬ 
ing of a primary ureteric tubule and the development of the 
secondary tubules. The primary tubule with its forked end 
presents an appearance very like the letter Y. The wall of the 
ureteric tubule is formed by simple, high columnar epithelium 
with large nuclei and deeply basophilic cytoplasm. The nuclei 
contain a rich chromatin network and one or more well-defined 
nucleoli. At the forking of the branches may be seen a large 
cell in mitosis. 
The ureteric branches of the second order continue to grow, 
their- cells proliferating by mitotic division. By the end of the 
second day in the allantois they in turn give rise to tertiary 
tubules. By further dichotomous division at the blind end of 
the newly formed tubules the ureteric tree grows and becomes 
more and more complex. Figure 4 shows the structure of the 
metanephros after two days’ growth in the allantois. A second¬ 
ary ureteric tubule ( 2') is seen dividing into two tertiary tubules 
(S'). These have grown to a considerable length. At the blind 
end of the tertiary tubule (S') a new subdivision is taking place, 
giving rise to tubules of the fourth order (4').' Subdivision of 
the primary ureteric branches goes on repeatedly in this manner, 
the branches so formed corresponding to the collecting tubules 
of the normal kidney. Each primary ureteric branch forms in 
this way a unit, the starting-point of a ureteric tree. The 
various lobules of the metanephric graft pictured in figure 7 
correspond to these units, each lobule constituting a primary 
ureteric tree. As seen from the study of the growth of the 
