194 Journal of Comparative Neurology. 



Supracommissure . — This is but weakly developed in Necturus. 

 As described by Herrick it consists of two bundles, a cephalic 

 larger and a caudal smaller which cross the meson indepen- 

 dently. Whether or not the smaller one could be regarded as 

 a commissura habcnanwi could not be determined since the two 

 parts of the commissure joined farther laterad. In addition to 

 a small contingent which seemed to pass ventrad into the thal- 

 amus, this tract passed ventro-laterad into the lateral wall of 

 the cerebrum. (Fig. 28.) 



Another small number of fibers, a few of them medullated, 

 passed down from the region of the habenas into the mesal wall 

 of the cerebrum. They did not however, seem to come from 

 the supracommissure. 



Ventral commissures. — Considerable difficulty attends the 

 homology of the fibers which cross in the floor of the dien- 

 cephal and mesencephal. Caudad and cephalad of the mesen- 

 cephalic pit myelinic fibers cross the meson; those caudad 

 are decussational, while those cephalad seemed to be decussa- 

 tional in part and may be, I am inclined to believe, regarded as 

 representing the commissia-a ansulata of teleosts. 



Caudad of the optic chiasma an extensive crossing of fibers 

 occurs, the larger portion of which, at least, are myelinic, in 

 which Herrick has attempted to find the representatives of the 

 commissures of the teleostean brain. The larger portion of the 

 fibers turn dorsad and then caudad to reach the mesencephal, 

 the commissura transversa {Decnssatio transversa, Edinger). I 

 cannot agree with Herrick in the recognition of a commissiira 

 Jiorizontalis ; what he figures and describes as such in Necturus, 

 I find to be amyelinic bundles from the thalamus which join the 

 basal prosencephalic tract. However, I agree with him in 

 being unable to find any trace of the " Mantelbiindel " of 

 Edinger ; a few only of the fibers which cross caudad to the 

 chiasma turn cephalad and suggest a possible homology, 



Precommissiire. — This has already been mentioned in the 

 discussion of the question of callosum. Fibers which cross in 

 this commissure turn, some cephalad, and some caudad, repre- 

 senting presumably the pars olfactoria and pars temporalis of the 



