250 H. F. Osborn 



extended by the union of the hemispheres , are seen transversely cut 

 upper and lower bundles, the latter consisting of two divisions, con- 

 stituting together the anterior commissure but quite distinct even in 

 the median line. These divisions considered, as the pars olfacto- 

 ria and pars temporalis. The upper bundle is the corpus cal- 

 losum. Opposite the letters vc is the foramen of Monro. 



Fig. 8. A transverse section of R. mugiens through the upper and lower 

 bundles. My sections are somewhat imperfect here and do not show 

 whether the pars temporalis can be distinguished in transverse section 

 or not. The upper bundle passes into the hemispheres just behind 

 the foramen of Monro. The transition from the fish to the batrachian 

 condition of the commissural ridge is seen in a comparison of sections 

 1, 13 and '. 



Fig. 9. Transverse section of a portion of the right hemisphere a few sections 

 in front of the foramen of Monro , showing the course of the corpus 

 callosum between the upper and lower cell groups and the distribution 

 of its fibres to the former. 



Fig. 10. Transverse section in the forward third of the hemispheres, showing 

 the reduction of the upper cell group and callosal tract. The distinction 

 between the pars olfactoria and peduncular tracts is slightly exaggerated 

 in the drawing. 



Fig. 11. Horizontal section in the level of the anterior commissure showing the 

 pars olfactoria traversing the nucleus in the brain stem, also the pars 

 temporalis and the inner olfactory tract. Below the latter there takes 

 place a partial decussation of the medial peduncular tracts. 



Fig. 12. Horizontal section in the level of the corpus callosum, showing its 

 course, distribution and relations to the foramen of Monro. The levels 

 of sections 8, 9 and 10 are given. 



Menobrauchus. 

 Fig. i'^. Sagittal section slightly to the right of the median line, showing the 

 complete separation of the corpus callosum and anterior commissure and 

 extensive ventriculus communis. Opposite vc is the foramen of Monro. 

 The superior commissure is more reduced than in the frog. The 

 pineal process is a closed sac. 

 Fig. 14. Transverse section through the corpus callosum and anterior commis- 

 sure. The latter is divided by a portion of the peduncular tracts. 

 The former passes separately across the ventricle. 



Emys Europaea and Testudo Graeca. 



Fig. 15. Vertical section to the right of the median plane. In this region the 

 fibres of the corpus callosum spread over the entire inner wall of the 

 hemispheres. Passing above the foramen of Monro is a smaller, com- 

 pact bundle cal'. The anterior commissure is undivided. By com- 

 paring this with fig. 13 the change of position with relation to the 

 foramen is at once apparent. 



Fig. 16. Transverse section through the corpus callosum and anterior commis- 

 sure in front of the foramen. The posterior branch of the corpus 

 callosum is shown just behind its union with the main tract. On the 



