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 larvce of Dytiscus and Acilius have figured as 

 types of the one-layer condition since the time of Grenadier'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, I. c, 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 nuclei 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 iheventral 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 directlj'' in front of the retina, to the surface of which they are 

 almost perpendicular. They consequently appear in the figure to form 



* Tlie "compound ocelli " are not so directly comparable with the types of eyes 

 witli which the present paper is concerned. 



t I can confirm the fact from my own observations. 



