No. I.] EYES OF ARTHROPODS. 20/ 



tion of the glass would show a decussation of fibres at the neclc 

 — and also at either end, provided the fibres were continued 

 onwards, — like that between the optic and retinal ganglion and 

 that between the inner and outer medulla. 



But there is another factor which renders the arrangement 

 of the fibres still more complicated. There are, besides those 

 fibres which run to the eye, others which leave the medulla in 

 the opposite direction, and go to the brain, perhaps to the eye 

 on the opposite side of the head. 



I have made some observations which render it probable to 

 my mind that each stalk of ganglion-cells in the optic gan- 

 glion divides at the surface of the medulla into two branches, 

 going in opposite directions. It is possible that of the two 

 sets of fibres thus formed, one, after its passage through the 

 medulla, goes to the eye, and the other to the brain, or to the 

 eye on the opposite side of the head. 



On pp. 226-7, Leydig (2), in speaking of the origin of the 

 nerves from various ganglionic centres, says : " Gegen diese 

 centrale Punktsubstanz richten sich die Stiele der Ganglion- 

 kugeln, um ihre fibrillare Materie dort beizumengen und aus 

 diesen centralen Herden von Punktmasse geht erst die einfach 

 streifige Substanz der peripherischen Nerven hervor." 



Dietl (4) supports Leydig's observation, and asserts that 

 fibres never pass directly from the ganglion-cells to the periph- 

 eral nerves ; the latter receive their fibres only from the net- 

 like " Marksubstanz." Claus (10), however, declares (p. 46) 

 that this view is untenable, since he has observed in Phronima 

 which is specially adapted to the study of this point, that fibres 

 arising from the cortical ganglion-cells pass diagonally through 

 the Punktsubstanz, without coming into any closer relation to 

 the latter substance, directly into the nerves arising from the 

 double ganglia of the ventral cord. My observations, as far 

 as the optic ganglion is concerned, agree with those of Leydig 

 and Dietl, for nerve-fibres running through the retinal ganglion 

 to the eye do not arise from the cortical ganglion-cells, but 

 from the depths of the medulla. 



There is apparently a wide discrepancy between Reichen- 

 bach's (24) recent observations on Astacus and my own on the 

 development of the eye and optic ganglion of Vespa. But 

 closer inspection will show, I think, that this difference is rather 



