136 PATTEN. (Vou. II. 
it is separated from the optic ganglion by the invaginated ecto- 
derm, z. After three or four more sections, we come to one 
(Fig. 39) that passes through the middle of the fifth eye and the 
lower end of the first. It should be remembered that the sec- 
tions are cut from an embryo a little older than that in Fig. 4, 
and that the posterior end of eye I. is on a level with eye V. 
We see that the continuity of the optic plate with the optic gan- 
glion is now interrupted everywhere except along three lines 
directly beneath, and parallel with, the median furrow of the 
eyes. These three connectives are short and nearly as broad 
as the optic plate. Compare Figs. 4 and 5a. They are the 
rudiments of the optic nerves, and are composed of a mixture of 
nerve fibres and ganglionic cells. 
If we refer to Fig. 1, and the sections of that age, Figs. 20- 
23, we shall see that the first cross section passes through the 
first ganglionic invagination, with whose direction of ingrowth 
it is nearly parallel. Now if we suppose a section to be cut 
through the long axis of the optic plate at that stage, we should 
then see three bands of cells—the optic plate, the middle wall 
of the ganglionic fold, and the optic ganglion itself. Owing to 
the way the cephalic lobes have developed, a cross section of the 
head shown in Fig. 4 will give a picture something like our 
imaginary one, only the middle wall of the fold has disappeared, 
and the inner one, the optic ganglion, is bent so as to appear 
like a thick, imperfect ring (Fig. 39). 
The brain at this period consists of a medullary core enclosed 
in a thick layer of ganglion-cells. The latter are continued, 
without any perceptible modification or break, into those which 
constitute the optic ganglion, which, up to this time, contains 
no medullary core. From this time up to the latest stages, 
there is no way of distinguishing the parts of the optic ganglion 
except by their position, and the nerves which go from them to | 
their respective ocular segments. 
I have represented two more sections from a head a little 
younger than that in Fig. 6. The first (Fig. 35) passes through 
the upper edges of the first and second ocular segments, and 
shows, besides eyes I. and III., the anterior edge of the optic 
ganglion. In the next section below, not represented in the 
plate, the optic ganglion is seen in section as a circular mass of 
cells, apparently cut off from the brain by the invaginated ecto- 
