162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



developed wherever the axons enter the ventricle and nowhere else sug- 

 gests that it has some function in connection with the optic reflex ap- 

 pai'atus. It doubtless serves as a support for the delicate axons, and 

 possibly has further functions, perhaps nutritive. 



From the base of the right ganglion habenula a group of axons 

 emerges into tlie diacoele and, uniting, pass caudad into the mesocoele, 

 where they coalesce with the other constituents of Reissner's fibre. 

 The discovery of the habenular constituents of Reissner's fibre throws a 

 flood of light on appearances observed in higher forms, but heretofore 

 difficult of int(irpretation. 



Johnston (:01, p. 155), after reviewing what is known of the ganglia 

 habenulae, says, "... The conclusion may be drawn that in the lower 

 vertebrates the ganglion habenula belongs to the central mechanism of 

 the parietal eye and the olftictory organ (the relation with the parietal 

 eye being the older ?)." It is evident that in the cyclostomes, where 

 the eyes are poorly developed, Reissner's fibre serves as a short circuit 

 for the transmission of reflexes arising from olfactory as well as optic 

 stimuli. In Myxine, which is blind, having only rudimentary eyes, 

 Reissner's fibre must be made up wholly of axons from the olfactory 

 centre in the ganglion habenula. 



II. Selachians. 



In the selachians the optic reflex apparatus reaches a high state of 

 development, perhaps, in the matter of the relative size of its elements, 

 the highest in any vertebrate. The nidulus of cells which gives rise to 

 Reissner's fibre is the most conspicuous element in the brain, and has 

 been known since its discovery by Rohon as the ' Dachkern,' or roof 

 nucleus. The cells lie in the median portion of the roof of the mesen- 

 cephalon, and usually extend through the greater part of its length, 

 though they vary considerably in their extent in different species. The 

 first differentiation of these cells is not apparent in selachians until a 

 much later period of development than in ganoids and teleosts. In the 

 oviparous species the apparatus is established about the time of hatch- 

 ing, but in tlio viviparous species it appears at a relatively later stage, 

 and is fully established only just before the animal attains a free life. 



A. Historical. 



Reissner's fibre was first seen in selachians by Stieda ('73), and some- 

 what later by Viault ('76) and Rohon ('77). All three agreed in regard- 

 ing it as an artificial coagulation product. .Sanders ('86) failed to find 



