93 



the dorsal commissure to the third ventricle and foramen of Monro 

 in different Amphibians may appear to differ essentially from the 

 typical arrangement seen in the Reptiles : but the difference in appear- 

 ance is merely the result of the varying development of the lamina 

 infraneuroporica and in no way interferes with the essential question- 



In these fishes which lack a cortex, of course there is no dorsal 

 commissure. The ventral or anterior commissure, moreover, in these 

 animals becomes separated into a number of strands — "the com- 

 missura interlobularis" — in the lamina terminalis. Since, according 

 to Edinger, a hippocampus appears on the dorsal aspect of the 

 Selachian cerebrum, one might expect to find a fornix- commissure 

 even among fishes. In birds the olfactory bulb becomes considerably 

 reduced in size and consequently the hippocampus, which forms part 

 of the olfactory centre, is correspondingly small. This accounts for 

 the small size of the dorsal (fornix) commissure, which, here as in 

 all other Vertebrates, connects the mesial surface (hippocampus) and 

 precommissural area (intraventricular lobe). When the olfactory bulb 

 is large, the dorsal commissure [the corpus callosum of Parker in 

 Apteryx ^] is correspondingly increased in size. 



In Mammals the dorsal commissure elongates horizontally with 

 the increased length of the hippocampus; and when the posterior 

 extremity of the hemisphere (including the hippocampus) bends down- 

 wards the fornix commissure takes a corresponding bend ^). 



The anterior or ventral commissure connects the olfactory bulbs 

 (the peduncles and prosencephalic part) and the corpora striata in all 

 Vertebrates. The tubercula olfactoria and pyriform lobes are also 

 connected by the same band, in all animals possessing these areas. 

 The whole of the first appearing "pallium" (in the Monotremata and 

 Marsupialia) is connected by the anterior commissure ; but the extreme 

 development of this part of the brain in the placental Mammal appears 

 to demand a shorter connecting path for its dorsal regions than the 

 infraventricular commissure affords. Fibres, serially homologous with 

 the latter, therefore take a supraventricular course as a corpus 

 callosum. These fibres probably belong morphologically to the lamina 

 infraneuroporica and supersede the anterior part of the fornix com- 

 missure, whose position they usurp. The corresponding region of the 

 hippocampus disappears and the supracallosal gyrus of Zuckerkandl 



1) Phil. Trans. Roy. Society, 1891. 



2) Proceedings Linnean Soc. N. S. W., Vol. IX (2. Series), Part 4, 

 635. 



