168 PATTEN. [Vor. II. 
optic plate, optic ganglion, and brain. The same feature is also 
shown in the arrangement of the eyes in the imagines of those 
insects supplied with frontal ocelli, for in such cases it is evi- 
dent, since I have shown in Vespa that the median unpaired 
ocellus is double, that the imagines have three pairs of eyes, 
and it at once suggests itself that there is some intimate connec- 
tion between this fact and the presence of the three larval seg- 
ments. It is possible that the solution of the problem, to which 
I shall return in my next paper on the “ Development of Acilius,” 
may lie in an explanation of the pupal stage. 
NEUROEPITHEL CELLS. — My studies on the “ Eyes of Mol- 
luscs and Arthropods” led me to believe that ganglion-cells were 
modifications of sensory ones. This belief was based upon the 
presence of intermediate stages between sensory and ganglionic 
cells, upon the constant occurrence of intercellular nerve ends, 
and upon the embryological evidence afforded by the fact that 
in Pecten the ganglionic cells of the eye and sensory papillz 
were derived from the cells of the hypodermic thickening that 
gave rise to these sense organs. This supposition further led 
to the conclusion that the optic ganglion of Arthropods could 
not be an outgrowth from the brain toward the eye, but one 
from the eye toward the brain. This conclusion is now in a 
measure confirmed by the history of the development of the 
optic ganglion of Acilius. 
When we review the semi-ganglionic cells described by me in 
Haliotis, “Eyes of Molluscs and Arthropods,” Pl. 30, Fig. 68, 
and in Acilius, Pl. X., Fig. 58, 4, and the myoepithelial cells of 
Ccelenterates as described by Hertwig, we perceive that nearly 
all these cells have three prolongations, one of which is di- 
rected outwards, and terminates between, the cells of the ecto- 
derm; the other two extend, from the opposite pole of the cell, 
inwards, and probably unite with similar prolongations from 
other ganglionic cells. There is no sharp line of demarkation 
between the tripolar cells described by me in the retina of Halio- 
tis and the neuroepithelial cells of Coelenterates as figured by 
the Hertwigs. In the case of Haliotis and Pecten, the origin 
of some, at least, of the ganglion-cells is beyond question, for 
they still form a part of the ectodermic thickening that gave 
rise to the sensory part of the eye. In Acilius we have tempo- 
rarily represented the condition that prevails in the sense organs 
