298 
HENRY ORR. 
mesoblast. In section, fig. 1, it may be seen that the thinner 
epiblast in the median line does not extend to the extreme 
anterior end of the rudiment of the nervous system, but that 
it ceases in the head region, while anteriorly the lateral 
medullary plates unite with each other undiminished in thick- 
ness, thus forming an anterior medullary plate (a. M. P.). 
The distinction between anterior and lateral plates is arbitrary 
and adopted only for convenience. They might be described 
as one thickened epiblastic plate, bent in such a manner that 
the curved part lay in the head region, while the two straight 
parallel ends lay one on each side of the dorsal median line. 
The distal periphery of the anterior medullary plate is a curve. 
Fig. 6 a represents a section through the anterior plate of the 
same embryo from which fig. 2 a was taken. It may be seen 
here that there is no sign of a bilateral division of the neural 
rudiment at its anterior end. 
The further development of the medullary plates in the 
dorsal region is shown in figs. 3 b, 4 c, 5 d, PI. XXVII. The 
lateral edges of the plates roll slightly upwards, forming the 
folds ( M.F . ). At the same time the median edges become 
pressed together, thus causing the floor of the median groove 
(G.) to sink farther inwards. The space between the medullary 
folds [M. F.) gradually decreases in size as the folds approach 
the median line. There is no very marked increase in the 
height of the folds. The originally dorsal surfaces of the 
medullary plates, bending inward, become pressed together in 
a vertical median plane under the groove, G. Across this line 
of median vertical contact there is no fusion of the cells. A 
heavy pigment marks this line (g., fig. 5 d) as continuous with 
the pigmented surface. Beneath the lower extremity of the 
line g. a small ridge of cells, continuous with the lateral halves 
of the neural rudiment, marks the original median connection 
of the medullary plates. By a comparison of the figures 
illustrating this period of development it may be seen that the 
cells of the neural rudiment gradually become smaller (owing 
to division and loss of yolk), that the whole organ becomes 
more compact and occupies much less space in the end than in 
