542 H. F. Osborn 



2).ol pars olfactoria of the anterior commissure, 



pt pars temporalis of the anterior commissure, 



rjin recessus pinealis, 



st.gr stratum grauulosum of the hippocampal fold, 



sul.h hippocampal sulcus, 



th optic thalamus, 



tr.o optic tracts, 



dc third ventricle, 



VC ventriculus communis, 



pre lateral ventricle. 



I olfactory nerve. 



// optic nerve. 



Tropidonotus natrix. 



Fig. 20. Transverse section of the forebrain trough the region of the commis- 

 sures, X 8. This shows the scattered callosal fibres, in connection 

 with which descend a number of fibres which are probably homologous 

 with the columnae of the fornix. 



Fig. 21. Horizontal section in the jjlane of the anterior commissure, x 8. It 

 shows the three divisions of the anterior commissure. 



Anas boschas, var. dom. 



Fig. 22. Vertical section through the lamina terminalis, x 8. This shows the 

 structures surrounding the third ventricle. In the roof are the po- 

 sterior commissure and the walls of the pineal stalk, surrounding the 

 recessus pinealis. The anterior wall is inducted with vascular 

 plexuses, part of which extend into the foramen of Monro. The corpus 

 callosum and anterior commissures have the same relative position 

 to the foramen as in the Reptilia. Above the optic chiasma is a 

 commissure, ban which may by homologous with the basilar com- 

 missure of Meynert. 



Fig. 23. Transverse section through the forebrain in the plane of tlie cerebral 

 commissures, x 4. This section shows the distribution of the fibres 

 of the diminutive corpus callosum in the reduced inner mantle of the 

 hemispheres; also of the anterior commissure, passing through the; 

 corpus striatum region and scattering into the cortical region of the 

 brain stem. Above the optic tracts and chiasma is shown the ba- 

 silar commissure (?) in its relation to the cerebral peduncles. 



Macro pus. 



Fig. 24. Transverse section through the forebrain of a specimen 3.(i cm long, 

 from the pouch, x 10. This shows the earliest condition of the cor- 

 pus callosum bundle and its position close to the anterior commis- 

 sure. Descending from the faint inner hippocampal fold with it are 

 the fibres of the fornix. Above, it divides into two parts, one, cat 

 extending forwards in the inner mantle, the other passing backwards 

 along the hippocampal fold. The vertical diameter of the section 



