196 BULLETIN OF THE 



On the question of the origin of the retina in arthropods, two un- 

 reconcilable opinions have been held. Some authors have maintained 

 that the retina was an outgrowth from the brain, and others that it was 

 a modification of the hypodermis. Graber may be taken as a represent- 

 ative of the former school, Grenacher of the latter. The evidence upon 

 which they based their opinions was derived in the two cases from quite 

 different kinds of eyes. Grenacher believed, since he had found in 

 eyes like those of the larval Dytiscus a retina which was continuous 

 with the hypodermis, that therefore the retina in the more complex 

 eyes was derived from the same hypodermal source. Graber, arguing 

 from those eyes in which the retina is separated from the hypodermis by 

 a preretinal membrane, maintained that the retina is an outgrowth from 

 the brain, and not derived from the hypodermis. Such an eye as the 

 larval eye of Dytiscus would, even in the absence of other evidence, 

 seriously weaken the force of Graber's argument. As an explanation of 

 such structures, Graber is inclined to think that the larval eye of Dy- 

 tiscus really possesses a preretinal membrane, with hypodermis in front 

 of it ; but that, on account of the thinness of this structure, Grenacher 

 has overlooked it. In other words, Graber considers the arthropod 

 ocellus as a two-layered structure, the outer layer of which is hypoder- 

 mal, and the inner layer, or retina, neural in its origin. 



In Graber's figures and description of the lateral eye in scorpions, the 

 two essential parts of the median eyes, the lentigen and retina, are rep- 

 resented ; but the lentigen, unlike that of the median eyes, is reduced 

 to a very thin layer of cells. This is perfectly consistent with Graber's 

 theory ; but whether it represents the actual structure of the eye or not 

 is questionable, since Lank ester and Bourne ('83, pp. 182 and 187) ex- 

 pressly state that the lateral eye of Androctonus is composed of a single 

 layer of cells, — a thickening of the superficial hypodermis, — and claim 

 that Graber is incorrect in describing a separate layer concerned in the 

 formation of the lens. 



Since the publication of these papers, Locy's discoveiy ui the method 

 of development in spiders' eyes has firmly established the hypodermal 

 origin of the retina. It has also offered a perfectly rational explanation 

 for the presence of Graber's preretinal membrane. Thus the hypothesis 

 of the neural origin of the retina is no longer tenable. 



The presence or absence of a lentigen and preretinal membrane is, as 

 Mark ('87, p. 55) has stated, important in determining whether a given 

 eye has been formed by involution with inversion or not. Although the 

 hypodermal nature of the retidas in both the lateral and median eyes of 



