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new form studied it became clearer that S ti e da' s upper bundle is not 

 homologous with the Pars olfactoria of the anterior commissure. This 

 and other observations led me to the following hypothesis: that the 

 lower bundle corresponds to the anterior commissure of 

 the mammalian brain including its two parts, while the 

 upper bundle is homologous with the Corpus callosum. 

 This view of these commissures in the amphibian brain seems to have 

 been held by Leur et and more distinctly byReißner; and in the 

 reptilian brain by Stieda*, who wrote as follows of Testudo and 

 Emys : »Der untere quere Theil der Commissur dürfte der sogenannten 

 Commissura anterior, der obere gekrümmte dem Corpus callosum im 

 Gehirn der Säugethiere zu vergleichen sein.« 



The material at my disposal for the solution of this interesting 

 question consisted of series of consecutive brain sections, cut in three 

 planes, of the following animals: an embryo Guinea Pig [Cavia); Turtle 

 [Testudo graeca, Emys europaea); Amphibians [Menopoma, Amphiuma, 

 Menohranchus^ Proteus^ Salamandra^ Mafia) ; Fish {Trutta f ano) . 



In the following outline of the results already reached, to avoid 

 confusion, I will continue to use the terms lower and upper bundles 

 and refer the reader to the foregoing description of their position, as 

 quoted from Stieda. 



1. Lower Bundle. Avery close examination of the brain of 

 the Frog [Rana mugiens), in vertical section, shows that the trans- 

 versely cut lower bundle consists of two parts, an upper larger part 

 making up Y5 of the bundle, a lower smaller part making up 1/5 of the 

 bundle. As the sections pass from the centre, the fibres composing 

 the (a) larger part turn forwards and traverse a mass of scattered 

 cells; from the latter they emerge and pass forwards and downwards 

 into the lower outer wall of the hemispheres towards the olfactory lobes. 

 This is a large, somewhat diffuse, tract whose fibres seem to supply not 

 only the olfactory lobes but the cells of lower ventricular gray sub- 

 stance of the hemispheres. The fibres of the smaller part (b) follow 

 the transverse direction of the above for some distance and then turn 

 backwards and outwards into the floor of the posterior horn of the 

 lateral ventricle. 



The course of these larger and smaller divisions at once suggests 

 that they may be compared to the Pars olfactoria and Pars temporalis 

 of the anterior commissure of the higher order of brain, and that the 

 disparity in size is owing to the feeble development of the temporal 

 lobe region in the amphibia. 



In the turtle the lower bundle arches strongly forward and un- 



* Über den Bau des centralen Nervensystems der Schildkröte. Zeitschr. f. wiss. 

 Zool. 25. Bd. 



