30 'Journal of Comparative Neurology and Psychology. 



branches, which together supply the whole dorso-lateral part of 

 the tectum with fibers, situated immediately under the opticus 

 fibers themselves. 



The lower and upper loops of the commissura transversa differ 

 in that the upper loop always consists of several distinct bundles 

 which farther caudad unite into a compact cap over the lower 

 loop. They may then be seen running on together into the sub- 

 ventricular cell layer the lateral part of which has already been 

 described by Fritsch as the nucleus corticahs (nucleus dorsalis 

 thalami of Goldstein). The group of cells situated under the 

 one just mentioned in which also fibers of this commissure end 

 a little farther mesad, was mentioned by Catois without any 

 special name as the homologue of Ramon y Cajal's "ganglion of 

 the commissura transversa" in the reptiles. As the latter layer 

 of cells may be seen extending almost to the nucleus rotundus (at 

 a somewhat higher level as it approaches the nucleus corticalis), 

 I have thought best to call this group of cells simply nucleus 

 prcerotundus . 



The method of the decussation of the commissura transversa 

 is rather complicated. A careful reconstruction convinces me 

 that the decussation takes place in three places. Several fibers 

 decussate far forward (Figs, xi, Plate I, and xxxiv, Plate II) and 

 probably it is this part which C. L. Herrick treats separately 

 from the rest as commissura ventralis. Another decussation of 

 the same significance occurs at a higher level (Fig. xxxvi). This 

 is the part which that author must have described as com. trans- 

 versa. Finally there are still other decussations between these 

 two chief parts of the commissure which relate parts of the second 

 decussation with the contralateral parts of the first (Fig. xxxv). 

 Afterward all the fibers run caudad together and terminate as I 

 have just described. This is the reason why I do not consider 

 this kind of a decussation as a commissure, and I am bound to 

 declare that they have nothing to do either with the tectum opticum 

 or with the corpus geniculatum laterale or even with the torus semi- 

 circularis, a name which Krause and many others have improperly 

 extended to include the whole region situated under the optic 

 ventricle, about which I shall state my opinion more fully later. 



There remain to be treated the -fihrcB ansulatce of Bellonci, 

 the commissura tuheris and the commissura horizontalis or com- 

 missure of Fritsch. 



