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 Lankester 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 discovery of 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 /entigen 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 retinas in both the lateral and median eyes of 
