VII] THE SENSE ORGANS 143 



The outer layer represents the original cuticle ; the inner laminae 

 are formed as a series of definite strata secreted by the corneagen 

 cells. These latter appear to degenerate rapidly after the com- 

 pletion of the lenses, and their nuclei cannot be made out in the 

 eyes of mature imagines. 



Beneath the corneal lens lies a very beautiful structure, the 

 crystalline cone or pseudo-cone (pc). This is a highly refractive 

 body of a vitreous appearance, in the form of a cone with its 

 apex inwards, and its broad outer end convexly rounded. It is 

 formed from four elongated transparent cells, two being larger 

 and two smaller (fig. 62 A, B). The nuclei of these cells lie close 

 up to their outer or distal ends, and are known as Semper' 's nuclei 

 (ns). These very peculiar nuclei appear to be filled with small 

 clear refractive bodies closely packed between large chromatophil 

 granules. Above each nucleus is a dense plate of deeply-staining 

 material. The inner end or apex of the pseudocone does not 

 actually end in a point, but the ends of the four cells stand slightly 

 apart, forming a slight hollow, into which the distal end of the 

 retinula is inserted. 



Embracing between them the inner tapering end of each 

 pseudocone are two large cells shewing a semi-annular form in 

 transverse section. These are the primary iris cells (fig. 62 c, ir). 

 Each has a large clear nucleus, embedded in a dense mass of black 

 pigment granules which completely fill the rest of the cell. Outside 

 these two cells are arranged numerous slenderer pigment cells of 

 very elongate form. These are the secondary iris cells (ir'). These 

 cells fall into two groups, according to the approximate positions 

 of their nuclei. One set lies around the pseudocone at a higher 

 level than the primary iris cells ; their nuclei are very clearly to 

 be seen lying between the inner halves of the pseudocones. 

 A section through the pseudocone at about the level of fig. 62 B 

 shews twenty or more of these secondary iris cells forming a close 

 ring around the pseudocone, which thus appears to be encased in 

 a ring of black pigment. Outside this ring other secondary iris 

 cells are arranged, while small pigment granules also form a kind 

 of hexagonal pattern around each pseudocone. Lower down, at 

 the level of the primary iris cells, the number of secondary iris cells 

 is much less, usually from six to ten (fig. 62 c). Passing next to 



