THE EYE AND OPTIC TRACT OF INSECTS. 
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structure with the substance of the epi-opticon and opticon 
shows that he was able to appreciate its histological nature as 
far as possible, and he did not fall into the error of supposing, 
as some subsequent observers have done, that it contained 
bacillar or cubical cell-elements. The nuclear layer and the 
“ nerve-bundle ” sheath of Berger together make up what I 
have termed above the “ terminal anastomosis.” 
Previous to the publication of Berger’s paper, the optic tract 
of Insects had been briefly described and names given to the 
various regions. Thus Weissmann (21) called the opticon and 
epi-opticon the “ bulbus,” the region where the optic fibrils 
decussate the “ Stiel,” and the peri-opticon the “Augen- 
scheibe.” Leydig (2) described the three ganglia in 
Formica as the “ kleiner Kern des Sehlappens,” “ grosser 
Kern d. S.,” and “ dritter Kern, d. S,” and in Dytiscus as the 
“erster,” “zweiter,” und “dritter” nuclei. Ciaccio (4) describes 
the following layers of the retina : 1. A “membrana limitans” 
posterior. 2. The layer of optic nerve- fibrils. 3. The layer 
of nerve-cells. 4. The “ membrana limitans ” anterior. 5. 
The layer of rods. In fact, Ciaccio anticipated Berger’s idea, 
that the retina of insects contains a number of nerve-layers as 
the Vertebrate retina does, but was unable to give it the 
minute histological analysis it deserved. 
Since Berger’s paper appeared Carriere (3) has described the 
peri-opticon as a layer of “ long, palisade-shaped cells, the 
number of which corresponds with that of the eye units. 
Every one of these palisade-cells possesses an oblong nucleus 
at its foremost somewhat broader end.” My researches show 
that this description is quite inaccurate. The elements of the 
peri-opticon are not cells, and the large oval nucleus situated 
in each element which Carriere figures does not exist. Nerve- 
cells, when they exist in the region of the peri-opticon in 
Musca, lie between the elements, and not in them, as my 
figures show. 
In a young Sarcophaga carnaria Carriere found “ a 
refracting chitinous or cuticular tube, which lies in the midst 
of the ‘ palisade-cell,’ ” and he finds in Musca vomitoria 
