No. 1.] EVES OF ARTHROPODS. 133 
cells, the cavity of which is continuous with that of the first 
ganglionic invagination. 
In the next two sections there is a solid chord of cells in the 
place of the V-shaped mass of Fig. 21. It connects the cells of 
the first ganglionic invagination with those of the second. 
The succeeding section (Fig. 22) passes through the second 
pit; e.oF 
That shown in Fig. 23 passes through the base of the antenna 
and the point marked g. v.3, Fig. 1. It shows at g. v.? a cross 
section of a thick chord of ganglion-cells extending from the 
third ganglionic segment to the under surface of the third 
ocular plate. It is produced by an inward proliferation the 
direction of which is parallel with the long axis of the antenna. 
The invaginated part of the optic ganglion now forms a con- 
tinuous semi-circular mass of cells tucked beneath the optic 
plate. J¢ zs directly continuous along its inner edge with the 
distal inner edge of the optic plate, p. n. 
This fact may possibly throw some light on the inexplicable 
presence of an optic, and ganglionic invagination side by side. 
My studies on Molluscs and Arthropods led me to suppose that 
ganglion-cells were derived from sensory cells that had wandered 
into the underlying tissues, leaving their outer ends, transformed 
into nerve fibres, sticking in the epidermis. Confirmation of 
this supposition was found in Pecten. Much better evidence will 
be given in describing the origin of the large tripolar cells in 
the optic ganglion of Acilius. 
But the point I wish to emphasize now, is, that from the ear- 
liest stages, the inner surface of the optic plate is continuous 
with the optic ganglion (Figs. 20-23, f. z.), and from what takes 
place later, it is highly probable that ganglion-cells are formed 
at this period by an inward proliferation of the optic plate. 
My idea is that an increase in the width of the optic plate, 
since its distal edge is fixed, would produce a fold like that 
in Fig. 20. The main part of the optic ganglion of this period 
may be regarded as a formerly sensory area with an underlying 
plexus of ganglionic cells continuous with a similar plexus under- 
lying the eyes. With the great development of the latter, 
all the cells of the sensory area were converted into gan- 
glionic ones, which were then overgrown by the optic plate 
and added to the plexus arising directly from the eyes. The 
