NOTE ON THE DEVELOPMENT OF AMPHIBIANS. 301 
as the anterior medullary fold. In both cases it forms the 
primitive morphologically anterior surface of the brain. There 
is a marked difference between this anterior brain-surface in 
the Lizard and the same part in Ambly stoma. In the Lizard 
the anterior brain-surface comes to lie at a right angle to the 
axis of the dorsal part of the neural tube, and faces posteriorly ; 
in Amblystoma it lies parallel to the axis of the dorsal part of 
the neural tube, and faces veutrally. This difference seems to 
be due to the different methods according to which in the two 
forms the medullary folds unite to form the medullary tube. 
In Amblystoma the condition is caused in the following manner. 
In the primitive neural rudiment there is a thinner median 
portion of epiblast lying between the dorsal medullary plates 
and behind the anterior medullary plate. As the distal lateral 
edges of the neural rudiment approach each other different 
effects are produced in the region of the thin median epiblast 
and in the anterior plate. In the first-named region, as the 
lateral edges of the medullary plates approach each other, 
their median edges are compressed, and as the width of the 
neural rudiment decreases its median thickness increases. In 
the anterior plate there is no thin median portion and no 
thickening resulting from compression, therefore as the lateral 
edges approach each other the median portion must bend 
downward. In this manner the median portion of the anterior 
plate comes to lie at a much lower level than the floor of the 
neural tube in the dorsal region. The cranial flexure is the 
result of the presence of an anterior medullary plate, and, as I 
have elsewhere pointed out, this seems to be the case also in 
the Lizard. 
In the Frog the anterior medullary plate forms a fold 
directly comparable to the medullary folds in the dorsal region. 
The anterior fold is, however, much more prominent than the 
folds in the dorsal region. Fig. 19 represents a median sagit- 
tal section of a Frog embryo at this stage. The lateral sections 
of this embryo show that the anterior fold (A. F.) is laterally 
and posteriorly continuous with the paired medullary folds, 
thus enclosing anteriorly and laterally the anterior enlarge- 
