— 42 — 



rhabdomeres are visibly narrowed proximal ly, which is not so 

 evident in other Dipt era. 



In the exterior optic ganglion of Diopsidae we find 

 the same layers of ganglion cells and fibres as in other 

 D i p t e r a (cp. schematic ifg). D To the basilar mem- 



brane is adjacent .Nervenbiindelschicht" of Berger which 

 consists of bnndles of nerve fibres („fibres postretiniennes" of 

 Viallanes, Fig. 3, /. j).). The spaces between separate 

 bundles are filled with an intercellular substance with few 

 nuclei, probably with haemolymph, and, besides, contain very 

 minute tracheae. The nerve fibres are directed from the om- 

 inatidia like the continuations of the rhabdom-cells, and pass 

 into the next layer — „Kornerschicht" of Berger (Fig. 3, sf. 

 gr.). This layer is composed of ganglion cells where the nerve 

 fibres seem to be dispersed and arranged into new groups. 

 The cells in this layer are very closely arranged, and are 

 provided with a round, small nucleus and a small quantity of 

 protoplasm. The next layer — the „Molekularschicht'' of lier- 

 ger (fig. 3, St. m.) is distinguished by a very regular stru- 

 cture which is in connection with the structure of the retina. 

 At small magnification, as in figs. 3 and 5, it is to be seen 

 that this layer consists of a row of „rods" arranged parallel 

 to each other, their number corresponding to that of the om- 

 matidia. The direction of the rods does not exactly correspond 

 to that of the ommatidia, as they diverge less than the latter. 

 In the distal part of the molecular layer, near the granular 

 eayer there is a single-rowed layer of elongated nuclei of 

 ganglion cells (cp. fig. D ); the nuclei are arranged 



between the dark-coloured „rods". Nothing has been mentio- 

 ned about such a single-rowed layer of nuclei in any of the 

 other D i p t e r a. I shall return to this question again. 



The „rods" of the molecular layer are seen in transverse 

 sections (cp. fig. 15, highly magnif.) as a bundle of about 

 seven dark-coloured fibres arranged in a circle. It is true that 

 the microscopical pictures are not very convincing, as it is 

 seen from the figure; however, 1 suppose that 7 — is a con- 

 stant number here. 



Dietrich, who had traced the course of the nerve fibre, 

 in the exterior optic ganglion of Diptera in greatest detail 

 considers this course in some species to be as follows. First of 

 all, be distinguishes two sorts of fibres: the fibres of the re- 



