478 EYCLESH YMER. 
embryo, or both. The facts recorded under IV show that an 
infolding occurs, while those given under III leave no doubt as 
to the backward extension of the embryo. 
When the blastopore has reached the circular form, there is 
_an infolding around its entire margin and a consequent exten- 
sion of the secondary area toward the ventral lip, so that the 
process of infolding varies at different points in the margin of 
the blastopore, the point of least activity being at the ventral 
lip. . 
Soon after the completion of the blastopore two areas of rapid 
growth are apparent on either side of the circular blastopore. 
These are the Ax/agen of the neural folds. A new factor is 
here introduced, which may explain how the blastopore changes 
from the circular to the linear form. 
The facts presented in the preceding pages lead to the 
following conclusions: 
The primary area of cell activity, at the upper pole of the 
amphibian egg, forms the basis of the cephalic end of the 
embryo. 
The secondary area of cell activity, on the blastoporic side 
of the egg, forms the basis of the greater portion of the posterior 
half of the embryo. 
These two areas constitute an embryonic tract, from which 
arise at least the anterior two-thirds of the embryo. 
The posterior end of the embryo is formed by a coales- 
cence of the lateral portions of the blastoporic margin. 
The greater portion of the embryo arises in the darker 
hemisphere by differentiation in situ, and not by concrescence. 
HuLL ANATOMICAL LABORATORY, 
UNIVERSITY OF CHICAGO, 
March 20, 1897. 
