300 
HENRY ORR. 
the parts in the dorsal and cephalic regions. It will be seen 
in the sagittal section (fig. 1) and in the transverse section 
(fig. 6 a) that at first the anterior medullary plate (a. M. P .) is 
externally flattened. In fig. 7 b, where the dorsal medullary 
folds have appeared (compare fig. 3 b), the lateral edges of 
the anterior medullary plate turn slightly upward (where 
the same letters are affixed to the numbers the 
sections are from the same embryo). In fig. 8 c this 
upward bending of the lateral edges has increased, and in 
fig. 9 n the edges meet dorsally. During this process the 
median part of the anterior medullary plate (A. F.) departs 
from its original slanting position (fig. 1), and comes to lie 
nearly parallel to the dorsal surface of the embryo, though at a 
lower level. The floor of the dorsal medullary groove extends 
forwards nearly horizontal as far as the region of the mid- 
brain ; there it bends downwards almost at a right angle, and 
joins the posterior edge of the medially horizontal anterior 
plate. Thus is formed the primary cranial flexure before the 
medullary folds have fused above the neural canal. In the 
head this fusion takes place later from behind forwards, curving 
down to the anterior edge of the anterior medullary plate 
(. A . F., fig. 9 d). The cranial flexure therefore is not simply 
a bend in the floor of the primitive neural tube, but is also a 
bend in the dorsal surface. It involves the anterior part of 
the neural tube in a bend about equal to a right angle. The 
line of fusion of the medullary folds in the head is homologous 
with the same fusion in the dorsal region. The morpho- 
logically dorsal surface of the neural tube extends therefore 
throughout the region of the fusion to the anterior edge of the 
anterior medullary plate. Taking into account the anterior 
bending of the axis of the neural tube, its morphologically 
anterior surface would be represented by the anterior medul- 
lary plate, which extends from the above-mentioned vertical 
portion of the floor to the anterior end of the dorsal fusion. 
The anterior medullary plate of Amblystoma is homologous 
with the anterior medullary fold of the Lizard, and for the pur- 
pose of indicating this homology I have marked it in the drawings 
