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 larva;, and their structure has been 

 discussed from a morphological point of view at great length. 

 Grenacher, Lankester and Bourne [229], and Carriere [232], 

 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, 



