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 retinulz 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 Carriére) 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 retine of many insects, but they 
are by no means universally present. In thin sections the fact 
Fic. 6y.—A semi-diagrammatic representation of the retina of the Blow-fly showing 
the various appearances seen in different preparations. 4, the normal appear- 
ance of the bacilli; 2, 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 ; Y, Tracheal network and cells, ‘the 
neuro-spongial network’ of Hickson ; » 6, membrana limitans externa ; m Z t, 
membrana limitans interna; 7, intermediate fibro-cellular reticulum; os, outer 
segments of the bacilli; 7s, inner segments of the bacilli; v, chaplet cells of 
Viallanes ; 7 /, Miillerian fibres ; 7 s, tracheal capillaries surrounding the inner 
segments of the bacilli; 4, 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 may be 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 
