No. 3.] VISUAL. CELLS IN GERTEBRATES. 5690 
found a fibrous structure in the molluscan visual cell, surmised 
a like structure in arthropods. Later, fibres were actually 
observed by Patten (’86) and others, and it was demonstrated 
by Parker (’95) that in the rhabdomes of the crayfish the 
fibrils composing them are neurofibrils and that the substance 
of the rhabdome is more correctly described as differentiated 
living material, comparable to the contractile substance of 
a muscle fibre, than as a secretion. This view, that the 
rhabdome is composed of neurofibrils, has been greatly 
extended for the invertebrates by the recent work of Hesse 
C2008 5,01): 
In the light of these discoveries in the lower animals it is 
not surprising that the attention of investigators should be 
drawn again to the more difficult problem of the finer structure 
of the rods and cones in vertebrates. That an interest exists, 
may be readily seen by reference to the literature of the last 
five years in comparison with that of the twenty years previous. 
Another incentive to research in this direction is to be found, 
no doubt, in the interest aroused by the discoveries of Apathy 
(97) as to the minute structure of the nervous system. The 
further exploration of the nervous elements through the 
researches of Bethe (’98), Prentiss (: 03), Schneider. (:.02), 
Parker (’95), Nissl (: 01), Embden (: 01) and others brings 
up, as a pertinent problem, the determination of neurofibrils 
in terminal sense-cells. Moreover, where these are found, is 
raised the question of their relation to other intra- and inter- 
cellular fibrils. 
Schneider (:02) is, so far as I know, the first author since 
the appearance of Apathy’s paper to claim an identification 
of neurofibrils in the rods and cones of vertebrates. He used 
the frog in his studies; concerning the longitudinal markings, 
about which there has been so much controversy, he says 
(p. 789) : “Das Sarc der Sehzellen ist zart langsfadig struiert; 
wir haben die leicht geschlangelt verlaufenden Faden als 
Neurofibrillen aufzufassen.” These neurofibrils he considers 
as continuous from the cell foot in the outer reticular layer 
