302 S. WATASE. 



Tlie central part always consists of a sinjjlc pjanj^'lion cell 

 (6^, Fig. n, ri. XXIX), and the jieripiieral of the rod-hearing 

 pigmented cells {A't, Fig. 6) which surround the former in tiie 

 centre. As to the minute structure of these elements I shall 

 speak later on. The cells composing the sensory portion of the 

 ommatidium send out from their proximal ends nervous processes, 

 which, forming a bundle, are seen to emanate from the basal end 

 of the ommatidium. ' The number of nerve fibres at the begin- 

 ning of the bundle and that of tlie cells composing the sen8f)ry 

 bulb of the ommatidium always correspond. These bundles, 

 however, soon break up as they penetrate deeper, and become 

 mixed up with the fibres from the neighboring bundles, forming 

 a complex plexus underneath the ommatidial area. Scattered 

 in the plexus are found a number of thickenings, which, in gold- 

 chloride preparations, appear as darkly stained masses of proto- 

 plasm occurring at the nodes of junction of tlie several inter- 

 crossing fibres (Figs. 44 and 45, PI. XXXII). In the fresh 

 state these enlargements contain a variable amount of yellow 

 granules imbedded in the mass of the protoplasm. In the 

 figures above referred to, mere outlines of such thickenings are 

 given. 



After the nerve fibres have formed the plexus they again 

 come out in bundles, and piercing through the perforations in the 

 basket-like chitinous support which underlies the ommatidial 

 region, travel forward, sligiitly inward, then downward and back- 

 ward until they terminate in the optic ganglion in the brain. 



b. The Structure of the Ommatidivm. 



In the general sketch of the compound eye of Limidus above 

 given, the structure of its morphological unit, the ommatidium, 

 has been briefly described. It consists of two parts, (1) the 

 senfiory and (2) the dioptric part, which is also partly protective. 



The sensory part consists of two factors, («) the central ele- 

 ment or the ganglion cell (6^, Fig. 6, PI. XXIX), and {b) the 

 peripheral elements or the retinulae (/i'<. Fig. 6). Tiiese two 

 kinds of cells are the only ones in which nerve fibres terminate. 



The retinula cells group themselves in the shape of a bulb, or 

 something as the several segments do in an orange, each segment 

 corresponding to a single rod-bearing cell or retinula (Fig. 10, 



