50 BULLETIN OF THE 



auch viber die Herkunft eimelner Augentheile, iiber die Abstammung der- 

 selben von der Hypodermis, namentlich bei den erstgenannten beiden 

 Catagorieii, kein Zweifel obwalten kann, so ist doch hier die Retina in 

 den von mir untersuchten Zustanden ausser aller Continuitat mit ihr 

 nnd jenen Augentbeilen, und der erforderliche strenge jSTachvveis dieses 

 jedenfalls hochst wahrscheinlichen urspriinglichen Zusammenhanges ist 

 erst noch zu fiihren." 



In his last paper on this subject Grenadier ('80, p. 430) reiterates his 

 inabihty 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 lat- 

 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 bj' 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 

 »io?iostichous.t 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 J 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." 



t " An ommateum consisting of a single layer of cells." 

 t See pp. 56, 57. 



