No. 440.] STRUCTURE OF OUTER SEGMENTS. 



543 



contractile substance of a muscle fibre, than as a secretion. This 

 view that the rhabdome is composed of neurofibrils has been 

 greatly extended anion- the invertebrates by the recent work of 

 Hesse (:oo, :oi). 



Since in some crustaceans the rhabdomes are not only fibrous 

 but are composed of plates not unlike the so-called plates in the 

 rods of vertebrates, it is natural to ask whether the rods in ver- 

 tebrates may not also be fibrous, and with this question the pres- 

 ent paper is chiefly concerned. The ease with which frogs could 

 be obtained at all seasons and the comparatively large size of 

 their rods led me to investigate the retinas of these animals and 

 I chose in particular the common leopard frog, Rana pipiens 

 Schreber, as a favorable species. 



For a satisfactory study of the rods it was necessary to free 

 them from their surrounding pigment. This was done in the 

 usual way by keeping the animal two or three hours in the dark. 

 In such "dark frogs" the retinal pigment completely withdraws 

 from the region between the rods into the bodies of the retinal 

 pigment cells. 



Of the various methods for obtaining un wrinkled retina' the 

 following was used with success, and I know of no other which 



