78 Journal of Comparative Neurology. 



tory fossa in the rhinencephalon of the alUgator and opossum.^ 

 This structure consists of an excavation of the mesal aspect of 

 the organ which is filled out by the fibres which enter there. 

 The whole pero is greatly thickened and the rhinencoel is thus 

 diverted laterad and describes an arch in both the horizontal and 

 vertical plane. The ventricle is not only thus reduced to a half- 

 moon shaped slit but the lateral walls of the tuber are thinned 

 to such an extent that they contain little but the epithelium of 

 the pes. 



A longitudinal section (Figs. 1-3, Plate VII.) is very in- 

 structive in showing that the attenuation extends ventrad and 

 dorsad so that the radix fibres emerge through these thin walls 

 of the lateral aspect. 



It is this delicate and one-sided structure of the tuber 

 which, taken in connection with the flinty brittleness of the 

 skull, makes the mutilation of the tuber almost a constant oc- 

 currence, explaining the incompleteness of earlier descriptions. 



This configuration is established before birth. The local- 

 ized development of the pero on the ventral aspect is perhaps 

 correlated with the isolated course of the highly developed tract 

 of Jacobson's organ. 



The structure of the tuber offers nothing peculiar other- 

 wise. The ganglion cells surround the cup-shaped fossa and 

 send their slender apex processes into the glomerule layer where 

 they enter into association with the olfactory fibre. The cen- 

 tral end of the nerve cells sends out short processes which break 

 up into dichotomous branches. The granules adjacent to the 

 ventricle are, as usual, of two sorts; larger pale nuclei with 

 slight protoplasm and which may be regarded as immature 

 nerve cells, and small dark nuclei which are perhaps migra- 

 tory corpuscles. 



In transections of the olfactory tuber of serpents [Tropidon- 

 otiis, Coluber, EiitcBuia) the fibres from the mesal aspect arch 

 dorso-laterad and collect with those of the lateral aspect and 



Journ. Comp. Neurol. Vol. IL Feb. 1892. 



