No. 3.] VISUAL CELLS TIN VERTEBRATES. 567 
the Neben-zapfen both ellipsoids and paraboloids. Hoffmann 
(*75) expressed doubt as to whether the double cones possess 
one or two nuclei, but inclines to the latter view. Schultze 
seemed to favor the former. He states (Schafer ’97, p. 49) that 
twin-cones possess two feet, one straight, the other oblique. 
The investigators whose results are summarized in the fore- 
going account carried the analysis of the structure of rods and 
cones so far that for twenty-five years almost no noteworthy 
advance was recorded. Many of the structural features dis- 
covered by them seemed hardly to accord with the conception 
of the rods and cones as modified nerve fibres. The plate 
structure of the outer segments was generally considered as 
strong evidence against this view. M. Schultze (’71, p. 257), 
however, seemed disinclined to give up the older notion. He 
says: “Es ist das Wahrscheinlichste, dass Nervensubstanz 
auch mit den Aussengliedern in Contact oder Continuitat 
stehe.”’ However, he admits (’72°, p. 1006) the possibility 
of a non-nervous function for the outer segment: ‘“‘Somit 
konnte moglicher Weise die Nervensubstanz mit den Innenglie- 
dern abschliessen und das Aussenglied einen nicht nervosen 
physikalischen Hulfsapparat darstellen.”” He considered it 
probable that the outer segment, through its laminz, serves as 
a reflecting apparatus and that the transformation of the lumin- 
ous rays into nerve impulses is effected in this region. These 
views were strengthened by his studies on invertebrates, where 
the laminated visual rods in mollusks and arthropods were con- 
sidered to be analogous to those in vertebrates. 
Many writers on invertebrates, especially on arthropods, 
e. g., Hensen (’65) and Watase (’go), believed that the visual 
cells were largely cuticular in structure. Thus Schultze’s 
comparison apparently led other students to the assumption 
that the outer segment of the vertebrate visual cell was in 
the nature of an “Abscheidungsproduct”’ or cuticular substance, 
and this view was held to be supported by embryological 
evidence. Although Schultze seems to have given expression 
