224 H. F. Osborn 



As the above theory differs widely from those which are now 

 current , it will be necessary to review the recent papers which 

 bear directly upon this subject without attempting to collect a com- 

 plete bibliography of the extensive past literature. In this review as 

 in the discussion which follows, are included the fishes, although 

 I have not as yet had the means of reaching any conclusions as to 

 the exact homologies between the cerebral commissures in this and 

 in the higher classes. 



Fishes. 



In the Selachian brain Miklucho-Maclay (1870, pag. 7), des- 

 cribed the union of the forebrain by a commissural system which he 

 compared to the primitive layer of the entire commissural system of 

 the higher vertebrates, viz. commissura anterior, fornix and corpus 

 callosum. He adds (pag. 29), that this homology had already been 

 suggested by Kollmann ' . 



RoHON (1877, pag. 31) without commenting upon Miklucho-Mac- 

 lay's suggestion describes, in the fore brain of the Selachians, a compact 

 transverse fibre system which be compares to the commissura anterior 



but does not state whether it consists of more than one distinct bundle. 



« 



Fritsch (1878, pag. 49, figs. 30, 31 and 38) gives a more precise 

 description of these commissures in the brain of the Teleosts (Scor- 

 paena and Serranus). They consist of three bundles: (1) The 

 uppermost is the inner olfactory nerve tract, the fibres of which form 

 a chiasma immediately in front of the commissura anterior, and which 

 is compared to the crossing olfactory tracts observed by Meynert. 

 Attention may be called to the fact that this tract and chiasma lie 

 in front and not above the anterior commissure, which Fritsch's 

 figures and description indicate plainly enough. (2) The second is 

 a strong bundle of white fibres which bends forwards, passing above 

 the cerebral peduncles upon either side into the lateral parts of the 

 brain stem, and representing the anterior commissure. (3) The third 

 bundle lies behind and somewhat beneath the second, bends back- 

 wards laterally, and cannot be homologized with the anterior com- 

 missure. 



Mayser (1882 pag. 271) gives a brief description of the anterior 

 commissure in the Cyprinidae as follows: »Der faserige Theil der 



Entwicklungsgeschichte der Adergeflechte. 18G1. pag. 11. 



