50 BULLETIN OF THE 
auch iiber die Herkunft einzelner Augentheile, iiber die Abstammung der- 
selben von der Hypodermis, namentlich bei den erstgenannten beiden 
Catagorien, kein Zweifel obwalten kann, so ist doch hier die Retina in 
den von mir untersuchten Zustiinden ausser aller Continuitat mit ihr 
und jenen Augentheilen, und der erforderliche strenge Nachweis dieses 
jedenfalls héchst wahrscheinlichen urspriinglichen Zusammenhanges ist 
erst noch zu fiihren.” 
In his last paper on this subject Grenacher (’80, p. 430) reiterates his 
inability to solve the problem, when he says: ‘‘ The genesis of the two- 
layer ‘Stemma’ out of the hypodermis, to which the conclusions from 
analogy point, is still entirely obscure to me also, and is only to be made 
out by direct observation.” 
Lankester and Bourne (’83) have expressed their opinion on the 
origin of the retina either in an incidental way or with a certain amount 
of reserve. I have not hesitated to class them on this side of the ques- 
tion, however, since they evidently incline in this direction. Of the dat- 
eral eyes in scorpions they say (p. 182): ‘Both nerve-end cells and 
indifferent cells of the lateral ommateum * apparently belong to the epi- 
blastic layer, and are shut off together with the layer of hypodermis cells 
from the subjacent connective tissue by a well-marked ‘ basement mem- 
brane,’ which in the region of the ommateum may be called the eye-cap- 
sule, or, better, the ‘ommateal capsule.” In this connection it should 
be borne in mind that these lateral eyes are claimed by them to be 
monostichous.— They believe (p. 211), however, that ‘‘a few examples 
clearly transitional between the monostichous and the diplostichous con- 
dition have been described by Grenacher (among Myriapods).” There- 
fore by inference their supposed diplostichous (in reality triplostichous) 
condition must likewise have had both its layers derived from the hypo- 
dermis. ‘The difficulties in the way of this transition from monostichous 
to so-called diplostichous eyes do not seem to have impressed themselves 
so forcibly upon these observers as they did upon Grenacher, who, not- 
withstanding his familiarity with the facts, confessed, as we have seen, 
that the double-layer condition presented a still unsolved problem. 
Finally, they have expressed { more precisely, although incidentally, 
the conviction that the retina in the central (“ diplostichous”) eyes of 
the scorpions is of hypodermic origin; but they have nowhere offered an 
* “All the soft tissues of an arthropod eye, as distinguished from the cuticular 
lens,” they call ‘‘ommateum.” 
+ ‘*An ommateum consisting of a single layer of cells.” 
t See pp. 56, 57. 
