62 RECORD OF CURRENT RESEARCHES RELATING TO 



The presence of the prseretiual lamella between the vitreous body 

 and the retina is an argument against Grenacher's theory of the 

 hypodermal (though it does not exclude the ectodermic) origin of the 

 arthropod retina.* 



The author's description of the retina differs in many important 

 respects from that of former observers, e. g. Grenacher. The optic 

 nerve, piercing the sclerotic, breaks up into a great number of fibres, 

 each of which is in direct connection with one of the visual cylinders 

 (Eetiuaschlauch or Eiidschlauch), a large globular nerve-cell with 

 vesicular nucleus and granular nucleolus occurring at the junction 

 between the two. In the distal or corneal end of each visual cylinder 

 is embedded a highly refractive rod, bluntly pointed at its distal end, 

 but at the opposite or proximal end breaking up into delicate fibrils, 

 which are probably directly continuous with the protoplasm of the 

 nerve-cell, and through it with the fibres of the optic nerve. Besides 

 the large nucleus in the nerve-cell, there is a nucleus in the corneal 

 end of the visual cylinder beyond the distal end of the crystalline 

 rod, and probably another in the middle of the cylinder. The 

 presence of these nuclei renders untenable the view of Grenacher that 

 each segment of the visual apparatus is the equivalent of a single 

 cell : they show that .at least two and probably three cells contribute 

 to its formation. Probably, therefore, the visual segment of the 

 single eye is not directly comparable with that of the compound eye. 



The visual cylinders do not all pass directly to the vitreous body, 

 or rather to the prasretinal lamella, and so lie in the direct path of the 

 light-rays ; those situated peripherally turn outwards, and end against 

 the inner surface of the sclerotic, of which they probably form the 

 matrix, just as the cells of the vitreous body are the matrix of the 

 prferetinal lamella. 



Another important point about the visual cylinders is that they 

 are not truly isolated structures, but are arranged in groups of five, 

 the distal pointed ends of the crystalline rods of each group bending 

 inwards or towards the axis of the group. Each group thus forms a 

 pentamerous, rctinula-like, perceptive organ, of higher order than the 

 single visual segment. 



In the course of his remarks on the retina, Graber enters upon a 

 comparison of a visual segment of the simple eye (Fig. 8, A) with 

 an acoustic segment (tympanalo Endschlauch) of the auditory organ 

 of Acridium (B). In both the nerve (») dilates into a nerve-cell {gz), 

 which is followed by a protoplasmic visual or acoustic cylinder, in 

 which are enclosed an anterior (vh) and a middle ()m^) nucleus, and 

 a rod-like body (s<) in direct connection, through the ganglionic cor- 

 puscle, with the nerve. The acoustic cylinder is continued distally 



* If the suggestion liere made by Graber turns out to be correct, the com- 

 parison between the arthropod and the vertebrate retina will be far simpler 

 than it is now assumed to be. For if the retinal rods of the simple eye of the 

 Tracbeata are not derived from the hypoderm or general epiblast, it is possible 

 that botli they and the fibres of the optic nerve may be formed from the cells of 

 a special ojitic outgrowth of the brain, and be thus, like the vertebrate rods and 

 cones, of indirect cpiblastic origin. But careful rmbrynlogical investigations are 

 necessary to settle the question, wliich is by no means decided by the presence 

 of the pr;er(tiual lamella. 



