The brain of Acipenser. JO 
a 
phibians, and to what EDINGER calls GuppEN’s commissure in 
Mammals. The first of the above mentioned bundles, which con- 
stitutes the greater part of the decussation in Acipenser, has not 
been mentioned by EDINGER, unless the tract of EDINGER & WAL- 
LENBERG in birds coming from the ganglion isthmi or from some 
more caudal source is this tract imperfectly traced out. The fibres 
of F. MAYER’s commissura transversa in Petromyzon, coming from 
the tectum and medulla, may be the same as the tractus tecto-lobaris 
and tractus lobo-cerebellaris et bulbaris in Acipenser, although they 
can not be certainly compared until MAyeEr’s full description appears. 
This review shows clearly that there are numerous decussations 
behind the chiasma which are not homologous throughout the Verte- 
brate series. One tract, the tractus tecto-lobaris, seems to be con- 
stant from fishes to Mammals. It has been described more or less 
completely in Petromyzon, Selachians, Acipenser, Amphibians, and 
Mammals. It has been traced and described in detail only in the 
present paper. In admitting its existence in Mammals I assume 
that it has been correctly traced by EDINGER, without attempting to 
decide whether it or the bundles described by KOLLIKER and others 
represent GUDDEN’s commissure. The character and homology of 
the Mantelbiindel in lower Vertebrates require further investigation. 
The decussation of the tractus lobo-cerebellaris et bulbaris was over- 
looked by EDINGER and has been described for the first time in the 
present paper. It should be found in all lower Vertebrates at least, 
and its large size suggests that it may be found in higher forms as 
well. 
In higher Vertebrates new elements appear in the postoptic 
decussation due apparently to the higher development of the thalamus 
optic centers. In Reptiles, birds, and Mammals a bundle crosses 
behind the chiasma which seems to be a true commissure between 
paired nuclei on the dorso-lateral surface of the thalamus, the corpus 
geniculatum mediale (?) Other thalamus nuclei in birds and Mammals 
contribute fibres to or receive fibres from the postoptic decussations. 
The nomenclature of these decussations is in a most unsatis- 
factory state. Until further investigations are carried out it is im- 
possible to make any general statement of homologies. I have 
pointed out that EDINGER has applied the name decussatio trans- 
versa to several bundles which differ widely from one another in 
origin and destination. Other authors have used various names and 
the confusion is as great as it can well be. In the present state of 
