416 
RUTH RAND ATTERBURY 
gradually into the looser mesenchyme which invests the whole 
anlage and which also occupies all the interstices between the 
ureteric branches. 
B. Growth of the metanephric grafts in the allantois 
It may be stated at once that both the ureter bud and the 
nephrogenous tissue proper survive and grow when grafted on 
the allantois. Indeed, the metanephric anlage of a seven-day 
chick embryo seems as fully capable of growth and differentiation 
in the allantois as in its normal environment within the organism. 
Figures 2 and 7 represent the metanephric anlage after seven and 
ten days’ growth, respectively, and illustrate to what extent 
growth and differentiation may proceed. Figure 2 is remarkable 
in showing that the metanephric anlage has developed in the 
form of two separate lobes, a larger and a smaller one. These 
two lobes, joined together by the main ureteric channel, have 
developed evidently from the cranial and caudal groups of ureteric 
branches present in the anlage at the time of grafting (text figure 
A). Figure 7 represents a section through a metanephros, the 
development and differentiation of which took place in the 
allantois. Almost all of this tissue has developed during the 
ten days’ growth in the allantois. The tiny metanephric anlage, 
which at the time of grafting was scarcely the size of a pin¬ 
head, has been transformed into a large, highly complex, lobu- 
lated organ very similar to the normal kidney of a chick embryo 
of corresponding age (seventeen days). 
The process of growth and differentiation of the metanephric 
anlage in the allantois was carefully studied and compared 
with the development of an undisturbed anlage at corresponding 
stages. For sake of clearness these observations will be recorded 
under separate headings: 1) Growth of the ureteric epithelium; 
2) Growth and differentiation of the nephric epithelium; 3) 
Development of the stroma. 
1 . Growth of the ureteric epithelium. The early stages of devel¬ 
opment of the metanephric structures in the allantois can be 
more easily followed by the study of the growth of the cranial 
