302 
HENRY ORR. 
meat of the neural groove ( F . B.). This anterior enlargement 
is the first rudiment of the vesicle of the fore- brain. The 
cranial flexure in this embryo is in process of formation ; when 
the medullary folds in the head later meet dorsally, the cranial 
flexure is complete. The presence of an elevated anterior fold 
in the Frog, and its absence in Amblystoma, is not so much 
due to absolute difference in the form of the neural rudiment 
as to the relative growth of the surrounding parts. In Am- 
blystoma the presence of the hypoblast and anterior end of the 
alimentary tract beneath the anterior medullary plate (fig. 9 d) 
prevents the latter from appearing as a fold raised above the 
head surface. But at a later period the hypoblast disappears 
from beneath the anterior plate, and the external surface of 
the anterior plate is then covered only with epiblast (fig. 12 e). 
The disappearance of the hypoblast and alimentary cavity 
from beneath the anterior medullary plate, or rather the 
(morphologically) anterior surface of the brain, is due to the 
more rapid growth of the brain, especially an increase of 
length, by which the fore-brain advances to a position in front 
of the anterior end of the alimentary cavity. At a very early 
stage the anterior end of the alimentary cavity is enclosed 
only by hypoblast and epiblast (Ep., Hyp., fig. 1). A fusion 
of these two layers soon takes place at this point, and indicates 
the eventual position of the mouth-opening. As the fore-brain 
is projected anterior to this mouth-fusion, the epiblast dorsal 
to the fusion is brought into close contact with the anterior 
surface of the bi’ain (fig. 12 e). Figs. 12 e and 13 e represent 
two nearly sagittal sections of the same embryo, one section 
passing through the oral fusion and hypophysis-rudiment, 
the other passing through the notochord and pineal rudiment. 
The age and general condition of development of this embryo 
will be best understood by comparing these sagittal sections 
with sections 14 f, 15 f, 16 f, which are horizontal and taken 
from an embryo of the same age. The anterior part of the 
alimentary canal is distended into a large pharyngeal branchial 
cavity (fig. 12 e). The hypoblast of the anterior wall of this 
cavity touches the nearly vertical floor of the fore-brain which 
