536 THE SENSES AND SENSORY ORGANS. 



observations, surrounds, but does not cover, the outer ends of 

 the retinal rods, which perforate the pigmented tissue and 

 terminate in juxtaposition with the membrana basilaris. If 

 these pigment cells are the representatives of the cells, the 

 nuclei of which are termed chaplet cells by Viallanes, as I 

 believe they are, the latter are clearly not nervous elements in 

 the sense in which the term is used by previous writers, 

 although the cells in question undoubtedly originate from the 

 retinal neuroblast. 



The Palisade Cells of Carriere These are undoubtedly the 

 structures which I describe as the proximal elements of the 

 retinulae. Carriere describes them as nucleated cells. Hickson 

 discovered that the nuclei lie in their sheaths, and this state- 

 ment is undoubtedly correct. They contain no nuclei, and 

 each consists of a bundle usually of seven fusiform enlarge- 

 ments of the fibres of the optic nerve. These elements, which 

 I formerly termed fascelli, are surrounded by a sustentacular 

 sheath, and each sheath exhibits a single nucleus. The susten- 

 tacular sheath is pigmented in Noctuids and consists of thread- 

 like fibres similar to the fringes of the retinal pigment cells of 

 Vertebrates. 



The Distal Segments of the Bacilli are usually very slender, but 

 expand at their outer ends ; they are highly refractive, and 

 frequently exhibit longitudinal striae in preparations fixed in 

 osmium peroxide. They are frequently curved and vacuolated. 

 The demonstration of these elements is difficult, but in suc- 

 cessful preparations their similarity to the outer segments of 

 the rods and cones of vertebrates is most striking, and every- 

 one who has worked at the histology of the vertebrate, retina 

 knows that in many Vertebrates it is by no means easy to 

 preserve these structures in a satisfactory manner. 



Like the distal outer segments of the vertebrate retina, these 

 structures resist most stains, but they can be coloured with 

 diffuse aniline stains; the coloration is, however, usually 

 evanescent. Such preparations should be cleared with xylol 

 and carbolic acid, as oil of cloves usually removes all traces of 

 the stain in a few minutes. 



