146 PATTEN. [Vot. II. 
the neurilemma as seen in different parts of the head. In Figs. 
42 and 43, its continuity with the basement membrane, and the 
drawn-out ectoderm cells are well shown, and in Fig. 45, the 
double membranes suspending each lobe of the brain. 
The same membrane forms a common envelop for the distal 
end of the optic ganglion and the roots of the nerves, but be- 
yond this point each nerve has a separate sheath. 
In the full-grown larve, the neurilemma is thicker and lami- 
nated, with small flattened nuclei in the middle of the layer. I 
have seen some cases when it appears to be composed of two 
thin membranes with nuclei between. 
ORIGIN OF GANGLION-CELLS. 
During the earliest embryonic stages, the whole inner, dis- 
tal edge of the optic plate is connected with the optic gan- 
glion by a mass of tissue composed of fibres mixed with ganglion- 
cells. The connection is gradually broken everywhere except 
beneath each eye, where broad bands of tissue remain forming 
the rudiments of the optic nerves. It will be more convenient 
to follow the development of the ganglion-cells in eye V., since 
the different stages of this eye have been described and illus- 
trated in most detail. 
Near the connecting bridge of cells in Pl. XL, Fig. 61, 2. g.c., 
the ganglionic nuclei are smaller, cell boundaries have disap- 
peared, and the tissue assumes an appearance more like that in 
the optic thickening. In fact, there is a gradual transition from 
the retinal cells to those of the optic ganglion. A careful exam- 
ination shows that among the half-ganglion, half-retinal cells, 
are many fibres of varying size, some large enough to be parts 
of slender cells, others so minute that they are seen with diffi- 
culty. The larger fibres are the outer ends of newly formed 
ganglion-cells; the minute ones are similar parts of cells whose 
outer ends have been converted into true nerve fibres. Sucha 
condition as this may be seen in sections of eyes from the stage 
shown in Fig. 4, Pl. VII., down to that in Fig. 1. In the older 
stages the tissue connecting the eyes and optic ganglion has 
become almost devoid of cells and is composed of closely packed 
fibres. The migration of sensory cells from the eye has nearly 
ceased, but one or two cells may still be seen, which, owing to 
their enormous size, offer special facilities for following the pro- 
