64 BULLETIN OF THE 
ation.* There are no satisfactory observations on the course of events 
during development in either of these cases. 
The simple eyes of the darve of Dytiscus and Acilius have figured as 
types of the one-layer condition since the time of Grenacher’s masterly 
work ; and indeed there seems at first sight little or no opportunity for 
any other interpretation, even though Graber (80) at first suggested, and 
then (in a footnote, 7. ¢., p. 67) definitely claimed the existence of a 
pre-retinal membrane in the case of Dytiscus. But the direct and evi- 
dent continuity of the “vitreous” cells with the retinal cells, especially 
the uniformity in the positions of the nueclec in the two regions, makes 
an inversion of the retinal layer extremely improbable. Even in the 
larger dorsal eyes of Acilius, where there is a perceptible difference in 
the size of the nuclei in the “ vitreous” and the retina, the continuity 
appears from Grenacher’s figure (/. c., Fig. 4) absolutely uninterrupted. 
There is a striking similarity between this eye and the anterior median 
eye of Salticus ; but the presence of (even a few) nuclei just in front of 
the anterior face of the retina in the latter case (compare Grenacher, ’79, 
Fig. 28) is sufficient evidence of an interruption in the continuity be- 
tween “ vitreous ” and retina in Salticus, and makes a substantial differ- 
ence between the two at least possible. However improbable a like 
interruption in the continuity of these cell-layers may be in Acilius, it 
is not to be overlooked that a complete separation of retina from 
“vitreous” even here could easily have been followed by conditions 
like those figured by Grenacher ; for to accomplish this it would only 
have required a subsequent displacement of the basal ends of the “ vit- 
reous” cells containing pre-retinal nuclei to the margin of the pigmented 
cylinder. That such a displacement — accompanied, perhaps, with 
partial obliteration — has really taken place in the case of Salticus, 
seems probable from the paucity of the pre-retinal nuclei figured,t and 
their entire absence from the funnel-shaped depression in the middle 
of the retina. 
Finally, in the ventral eye of Acilius figured by Grenacher (’79, Fig. 10), 
the appearance of the vitreous is certainly not more favorable to a mono- 
stichous than to a so-called diplostichous condition. While in the dorsal 
eyes the basal (nucleated) ends of the vitreous cells abut upon the peri- 
phery of the cylindrical ocular mass, in the ventral eyes they appear to 
end directly in front of the retina, to the surface of which they are 
almost perpendicular. They consequently appear in the figure to form 
* The “compound ocelli” are not so directly comparable with the types of eyes 
with which the present paper is concerned. 
+ I can confirm the fact from my own observations. 
