375 



part of the cell. The four pigment cells (^) surrounding the 

 ommatidium have altered in shape ; they appear now as rather 

 thin filaments, with a great swelling at a distance of about 

 one-third their length from the anterior end, a swelling which 

 lodges the nucleus (fig, 59). The latter is rather distinct; its 

 chromatin is scattered, and it contains a small but very dis- 

 tinct nucleolus. 



From now onwards the visible changes become more pro- 

 nounced. The rhabdome and sheath cells continue to grow 

 in length, reaching in the twenty-one hour pupa a length of 

 about 38/x, the total thickness of the eye at this stage being 

 48/x. The four vitreous cells have increased in size, and now 

 quite surround the end of the rhabdome cell except at its 

 termination, where it can still often be seen projecting beyond 

 them (fig. 61). Their cytoplasm is fairly clear, and they 

 possess each a relatively very large, rather irregular nucleus. 

 The two lens cells, which have been gradually approaching 

 the vitreous cells from the sides, now wholly surround them 

 above and at the sides, embracing them closely (fig. 62). The 

 nuclei are lodged in their lower portion; each possesses a 

 very distinct nucleolus. The rhabdome cell has meanwhile 

 been narrowing for the greater part of its length, but it is 

 even yet visible, though only very faintly, amongst the 

 vitreous cells. At this stage the sheath cells which have 

 extended by very delicate processes, over the proximal filament 

 of the rhabdome cell, begin to develop granules of a reddish- 

 brown pigment throughout their length, so that in this insect 

 the capacity of forming pigment is not confined to the true 

 pigment cells (fig. 63). The latter, indeed, at this stage are 

 still quite devoid of granules. Other changes, however, which 

 may be the forerunners of pigment formation, are now gfoing 

 on in them, and they now assume a very remarkable form; 

 the cells which were, before this, filamentous, or at times 

 spindle-shaped, become even narrower, except in their distal 

 third, which remains rather thick (fig. 62); their protoplasm 

 becomes vacuolated, the vacuoles at times producing small 

 swellings in the thread-like filament. It is towards the 

 proximal (inner) end of the cell, however, that this process 

 has its most remarkable result. Here, at a short distance 

 from the end, a relatively huge vacuole is formed which causes 

 this part to swell up in a large globule. This condition is 

 very characteristic of the pigment cells at this stage. During 



(3) At first sight, there appear to be six pigment cells sur- 

 rounding each ommatidiuni. Each pigment cell, however, becomes 

 applied to two ommatidia; hence of the six which are observed 

 around each ommatidium, two ' 'belong" to adjacent ommatidia. 

 This will become clearer by examining fig. 64d. 



