132 PATTEN. [VoL. II. 
Between the brain and the optic plate (Fig. I.) is a broad 
expanse of ectoderm composed of faintly stained cells contain- 
ing coarsely granular, or even flocculent, protoplasm and large 
spherical nuclei. 
In Pl. IX., Fig. 19, which represents a cross section through 
the middle of the cephalic lobes of an embryo considerably 
younger than that shown in Fig. 1, there is no thickening to form 
the brain, although the distal edges of the lobes have already 
given rise to the optic plate, on the inner edge of which is a 
depression, containing large wedge-shaped cells quite different 
in appearance from those on either side of them (0. g.). This 
depression, the beginning of the invagination of the optic gan- 
glion, is fairly uniform in depth, and in surface views appears 
like a semi-circular furrow on the inner edge of the optic plate. 
The ectoderm that gives rise to the optic ganglion divides into 
three segments, or lobes, each of which ts united on the one hand 
with a segment of the brain, and on the other, with a segment of 
the optic plate (Fig. 1, 0. g.*"). 
The semi-circular groove is soon deepened to form two distinct 
pits with slit-like openings (Fig. 1, g. v.t*). There is no infold- 
ing between the third segment of the optic plate and the third 
segment of the optic ganglion, but there is, as shown in sections, 
a distinct inward proliferation of ganglion-cells at this point. 
Each segment appears in some cases to be divided by a faint 
line into two parts, a condition that may have some connection 
with the fact that it belongs toa part of the optic plate provided 
with two eyes. 
Figs. 20-23, Pl. IX., represent four cross sections of the 
cephalic lobes during the stage shown in Fig. 1. The first 
section passes through the first ganglionic invagination, g. v.}, 
the lateral wall of which is composed of a single layer of loosely 
connected cells, continuous, at the opening of the pit, with the 
edge of the optic plate. There is no infolding between the 
anterior edges of the optic plate and first ganglionic segment 
(Fig. 1). The invagination appears just behind this point as a 
slight furrow that increases in depth backwards as far as the 
anterior end of the second segment of the optic plate. 
The section shown in Fig. 21 passes through the bridge of 
ectoderm separating the first invagination from the second. 
Beneath the uninfolded layer is a V-shaped mass of ganglionic 
