No. 1.] EYES OF ARTHROPODS. 145 
It will be seen that in the later stages the medullz show 
more clearly their primitive relations to one another than do the 
eyes. At this period, it is impossible to recognize which eyes 
belong to the same segment, or that there is any paired arrange- 
ment at all. 
NEvuRILEMMA. — The brain, optic ganglion, nerves, and eyes 
are suspended or enclosed.in a common envelope continuous 
with the basement membranes and derived from the ectoderm. 
During the youngest embryonic stages, the inner surface of the 
ectoderm, especially of the thickenings that give rise either to the 
eyes or nervous system, is covered by a delicate membrane 
which in the majority of cases is quite devoid of nuclei. When 
the eyes are invaginated, the membrane is pushed in likewise, 
but it is unaffected by the shifting of the cells to form the 
corneagen and the outer wall of the optic vesicle, and is con- 
tinued from the indifferent ectoderm over the bulb of the eye 
to the optic nerve. 
The neurilemma of the brain and optic ganglion is formed in 
a similar manner. As the ectodermic thickenings that give 
rise to these organs separate from the parent layer, they are 
suspended in a sort of sling formed by the basement membrane, 
which is distinctly nucleated where it surrounds the developing 
organs. As the latter separate from the ectoderm, the mem- 
brane surrounds, but does not completely enclose them, for 
at quite late periods one sees at certain places the two limbs 
of the membrane close together, suspending the brain, as 
the intestine in its mesentery, to the wall of the head (Pl. X., 
Figs. 43 and 44). Wherever the membrane is still attached to 
the surface ectoderm, the cells are drawn out into long fibres 
with nuclei at various heights. Some cells are entirely sepa- 
rated from the others, taking up a position on the outer surface 
of the membrane, in the formation of which they seem to take 
a part. As the outer surface of the basement membrane be- 
comes the inner one of the neurilemma, we find that the nuclei 
of the latter, during embryonic life at least, are on its inner sur- 
face (Figs. 42 and 47). 
In the adult larvze, the suspending membranes are reduced to 
cords of fibres mixed with nuclei extending from the posterior, 
median edge of each brain lobe to the roof of the head. 
In Figs. 42-47, are shown different stages in the formation of 
