98 PALTEN. [Vo.. II. 
edge of the plate, are infolded, forming an almost semicircular 
depression, the beginning of the invagination which subsequently 
gives rise to a large part of the optic ganglion. 
The optic plate is soon divided into three segments (Pl. VII., 
Fig. 1). The anterior, or first segment, forms a kind of corner to 
the anterior, lateral edge of the cephalic lobe; the second occu- 
pies the median and larger part of the optic plate, with which its 
long axis is parallel; and the third forms the posterior edge of 
the cephalic lobes, its long axis being nearly at right angles to 
that of the second segment. The distal edge, and in some 
cases the whole, of the plate is buried in the yolk; it is then 
necessary, if one desires to obtain surface views, to cautiously 
pick the yolk away from the lobes with needles. 
The ganglionic invagination soon deepens, and also shows 
traces of a differentiation into three parts, one of which lies on 
the inner edge of each segment of the optic plate (Fig. 1 0.¢.)°). 
From the median part of the cephalic lobes rise three pairs of 
thickenings, one pair opposite each ganglionic segment. They 
form the rudiments of the brain (Figs. 1-3, 41%), and appear to 
be direct continuations of the segments of the ventral chord. 
The embryo is divided by transverse folds into a certain 
number of post-oral terga, the first three of which are grad- 
wally drawn towards, and united with, the cephalic lobes. The 
line surrounding the optic plate, after the rupture of the embry- 
onic membranes, is not the old boundary of the cephalic lobes, 
but a new formation, which includes the cephalic lobes and the 
terga of the first three post-oral segments (Pl. VIII., Fig. 8). 
The indentation; zg.>", appears in surface views like a newly 
developed eye. It was quite enigmatical until a more careful 
study showed it to be the boundary between the posterior edge 
of the cephalic lobes and the terga of the first three post-oral 
segments. The last trace of this fold disappears soon after the 
stage shown in Fig. 8. 
While the optic plate is still in its semicircular form, two 
areas in each segment become visible in surface views, on 
account of the peculiar arrangement of their nuclei. From 
each of these areas is developed an ocellus, which I have 
numbered in the way shown in the plates. The growth of the 
embryo gradually forces the median, ventral part of the germ 
band forwards, leaving, as it were, the distal ends of the cephalic 
