534 



THE SENSES AND SENSORY ORGANS. 



ends of the rods difficult, as in all but very thin sections these 

 cells cover either the upper or lower surface of the rods. 



The Chaplet Cells of Viallanes. The outer segment of each 

 retinula is only about half the diameter of its inner segment, 

 and the interspaces between the retinulas are filled with these 

 cells ; so that the retina appears in many sections to consist of 

 two layers, an inner palisade layer (the layer of Carriere) and 

 an outer layer of cells, the round nuclei of which bear so close 

 a resemblance to those of the nerve cells of the optic ganglion 

 that one is tempted to believe that they are nerve elements. 

 Similar cells are seen in the retinae of many insects, but they 

 are by no means universally present. In thin sections the fact 



V 3- D ff 



FlG. 69. A semi-diagrammatic representation of the retina of the Blow-fly showing 

 the various appearances seen in different preparations. A, the normal appear- 

 ance of the bacilli ; />, spaces from which bacilli have disappeared, showing the 

 connective reticulum and the chaplet cells of Viallanes ; C, the connective 

 reticulum only, showing the flat surfaces of the connecting bands between the 

 internal and external limiting membranes ; />, Tracheal network and cells, ' the 

 neuro-spongial network ' of Ilickson ; m t>, meinbrana limitans externa ; in I /, 

 membrana limitans interna ; /, intermediate fibro cellular reticulum ; o r, outer 

 segments of the bacilli ; is, inner segments of the bacilli ; 7% chaplet cells of 

 Viallanes ; tn f, Miillerian fibres ; ;/ s, tracheal capillaries surrounding the inner 

 segments of the bacilli ; //, cells connected with the finest tracheal capillaries. 



that these cells merely surround the bacillary elements is easily 

 demonstrated. 



There are two views with regard to the nature of these cells. 

 They maybe the representatives of the pigment cells (tapetum) 

 which surround the outer ends of the rods in many insects; as 

 pigment is absent or very feebly developed in this region in the 



