BY G. ELLIOT SMITH. 651 



callosum of most writers on Submammalia, is nothing else than 

 a hippocampal commissure — a view, which a comparison with the 

 arrangement of the commissures in Platypus would lead one to 

 expect. The great source of confusion to investigators not only 

 upon the Marsupial but also upon the Sauropsidan cerebrum, in 

 attempting to compare the latter with the placental mammal, is 

 the altered position of the hippocampal commissure — firstly, by 

 the bending of the hemisphere accompanied by a corresponding 

 bend in the commissure, and secondly, the disappearance of just 

 that part of the hippocampus and its commissure, which clearly 

 maintains its true morphological position as the upper margin of 

 the choroidal fissure, dorsal to the septum pellucidum. In the 

 Platypus we have the key to unlock the whole question. 



The following summary presents a hypothetical explanation of 

 the presence and order of appearance of the three commissural 

 bands met with in the region under consideration. 



In the lowest stage of cerebral development the anterior com- 

 missure is the great commissural system of the cerebrum. 

 Included in the anterior commissure are numerous strands of 

 varied significance, which may in lowly forms be quite distinct 

 from one another as they lie in the lamina terminalis, e.g., in 

 Teleostei, where this anterior commissure system is known as the 

 " cotnmissura interlobularis" (Gottsche). Early in development 

 the upper border of the choroidal fissure becomes differentiated as 

 a hippocampus, which, before the ventricle becomes bent, lies 

 entirely dorsal to the fissure, as in Platypus and the Sauropsida. 

 This is the portion of the cortex furthest removed from the 

 situation of the anterior commissure, and hence this — the hippo- 

 campus — is the region which first demands a shorter route for its 

 connecting fibres. Hence the hippocampal commissure appears 

 as the second distinct commissural band, and we have the Mono- 

 treme and Marsupial condition. 



In the ontogeny of the Eutherian brain this oi'der of develop- 

 ment of the commissures is also observed, the anterior commissure 

 being the first to appear, and later the combined calloso-hippo- 

 campal commissure. Of the latter commissure, the commissural 



