512 THE SENSES AND SENSORY ORGANS. 
coloured pigment, which rapidly assumes a yellow colour after 
death. 
The Preretinal Membrane.—The retina is separated from the 
vitreous by an extremely thin membrane, which is seen in some 
of my sections to be continuous with the external pigmented 
sheath; but it is not pigmented, and is so thin that it is only 
occasionally capable of demonstration; except at its edges 
it appears to be a mere cuticular layer. Graber [223] and 
Lankester and Bourne [229, p. 189] describe this preretinal 
layer as continuous with the retinal capsule. 
b. On the Morphology and Comparative Anatomy of Simple 
Eyes in Arthropods. 
The ocelli of imaginal insects are exceedingly similar to the 
simple eyes of Arachnids and Myriapoda, and to the lateral 
simple eyes of many larve, and their structure has been 
discussed from a morphological point of view at great length. 
Grenacher, Lankester and Bourne [229], and Carriére [282], 
have held that the retina is derived from the hypodermis; and 
Locy [240] and Mark [241] believe that they have seen evidence 
of an actual invagination from which the retina originates ; on 
the other hand, Graber [223] and Schimkewitsch [234] state 
that the retina is developed as an outgrowth from the cephalic 
ganglia, an opinion which I have long maintained. 
Although it must be admitted that it does not follow that all 
simple eyes in Arthropods arise in the same manner, it is most 
probable that they do, and I believe that many unnecessary 
complications have been introduced into the discussion of their 
origin by the attempt to prove that the retina originates from 
the cutaneous epiblast. 
Lankester and Bourne [229] have introduced the terms 
‘monostichous’ and ‘diplostichous’ to designate stemmata, in 
which they believe that a single or a double layer of cells 
exists beneath the cornea. They maintain that the lateral 
eyes of Scorpions have a single layer, the retina, behind 
the corneal lens; whilst the median eyes have a double layer, 
