THE AMERICAN A' A TURALIST. [Vol. X XXVII. 



the distal end of the segment than at the proximal one, the dif- 

 ference having been attributed to a protecting sheath over the 

 latter. The presence of an axial fiber, as maintained by Ritter 

 and others, was discredited because of much negative evidence. 



In contrast to this general view Krause ('92), whose compara- 

 tive studies included particularly Rana and Salamandra, main- 

 tained that the outer segment consisted of a bright " Grundsub- 

 stanz," in the periphery of which fibrils were imbedded and 

 twisted in so close a spiral that they gave the appearance of 

 transverse striations, thus producing a condition favorable to 

 apparent transverse fracture. Greeff (:oo, p. 103), however, 

 has recently expressed himself in favor of the older view that 

 the outer segment consists of a series of plates with an envelop- 

 ing sheath. "Jedes Aussenglied besteht 1. Aus einer Hiillc 

 (Mantelschicht, Rinde, Haut) und 2. einem aus Plattchen 

 und Zwischensubstanz gebildeten InhaltT This view is also 

 accepted by Levi (:oi). 



Bernard (: 00, :oi), who has published an account of the 

 embryonic as well as of the adult rods in amphibians, has main- 

 tained that these structures arc protrusions from a syncytial 

 retina and that each rod is a delicate protoplasmic vesicle 

 traversed by a reticulum which eventually becomes condensed 

 into the axis of the rod by the absorption of a colorless 

 refractive and amorphous matter from the pigment cells. Thus 

 even among recent investigators much difference of opinion 

 exists as to the structure of the vertebrate rods. 



The progress made in the study of the rods in vertebrates 

 during the last twenty-five years stands in considerable contrast 

 with that made in the investigation of the terminal optic organs 

 of invertebrates. In the arthropods, for instance, the rhabdomes, 

 the analogues of the rods and cones of vertebrates, were sup- 

 posed by most of the earlier writers to have been formed by 

 secretion, and in fact Watase ('90) went so far as to compare 

 them with surface cuticula. Their fibrous character, however, 

 was observed by Patten ('86) and others and it was demonstrated 

 by Parker ('95) that in the crayfish the fibrils composing them 

 are neurofibrils, and that the substance of the rhabdome is 

 more correctly described as differentiated living material like the 



