222 



patten: . [Vol. I. 



cells, the 02itcr zvall of the optic vesicle. Beneath the centre of 

 the lens this layer is seldom visible ; it is thickest on the pe- 

 riphery of the ommateum, where it contains a single layer of oval 

 and faintly-stained nuclei. It was only on very successful sec- 

 tions that this layer could be observed at all. 



The inner layer, or ommateum, consists of well-developed 

 ommatidia, each one composed of at least nine cells. The 

 centre of each ommatidium is occupied by three retinophorce , 

 each one bearing a flattened rod, broad at its outer extremity, 

 and running to a point at the opposite end. The axial faces of 

 the three rods unite to form a cone-shaped body, with the base 

 directed outwards, and giving either a T-shaped or triradiate figure 

 in cross-sections. The ends of the three arms of the figure are 

 indented, while in the centre of the figure is a triradiate mark, 

 which indicates the boundaries of the rods. The nuclei of the 

 retinophorae are probably situated over the outer end of the 

 rods, although this is a point which I have not determined with 

 certainty. 



The retinophorae are surrounded by six pigmented cells 

 arranged in two circles. The outer circle is formed by 

 slender cells, one in each angle of the trifluted cone. Their 

 hardly distinguishable nuclei are situated about opposite the 

 middle of the cone, while their inner ends terminate in long 

 hyaline bacilli, similar to those of the outer pigmented cells in 

 Pinaeiis. The cells of the inner circle are much larger, and con- 

 tain large oval nuclei, situated below the inner ends of the rods. 

 At first sight they appear to be the only nuclei present in the 

 ommateum. The thickened edges of the outer ends of these 

 deeply pigmented cells are folded over the flutings of the cone. 

 In isolating these cells, the median part of their outer ends is 

 so thin that it often ruptures. Such cells then appear to be 

 forked. In cross-sections there appear, at first sight, to be 

 nine pigmented bands, surrounding the outer end of the cone. 

 There are, however, only six, since the two bands, one on either 

 side of the fluting, belong to a single cell. 



The three rods formed by the retinophorae terminate inwardly 

 in three slender fibres, very much like the slender inward pro- 

 longation often formed by crystalline-cones in the compound 

 eye. This fibrous prolongation of the rods is enclosed in a 

 delicate tube, formed by the coalescence of the inner ends of the 



