184 G. ELLIOT SMITH. 
to describe. In this aceount, therefore, such brief references 
will be made to the histology of the adult brain as are neces- 
sary to a proper understanding of the foetal structures. 
Forming as it does the link between the Mammalia and the 
lower Vertebrates, Ornithorhynchus occupies a position of 
supreme importance and interest to the morphologist. This 
importance is enhanced in the case of the brain, because this 
organ presents a number of very significant transitional fea- 
tures, several of which shed an important light upon the 
morphology.of the cerebrum in the whole Vertebrate series. 
In addition there are a number of purely “ individual”? fea- 
tures—notably the enormous development of the trigeminal 
nerve and sensory tract—which give added interest to the 
study. Although in the histological differentiation of its parts, 
and in their general arrangement, the cerebrum of Platypus 
clearly conforms to the mammalian type, yet numerous fea- 
tures—such as the arrangement of the hippocampus, the dis- 
position of the structures in the lamina terminalis, and the 
“ precommissural area”’—indicate its close Sauropsidan aflini- 
ties. It is of interest to note that the resemblances to the 
lower brain are even more marked in the foetus under con- 
sideration than in the adult. 
General Account of the Fore-brain. 
At the outset one is immediately confronted with the difficul- 
ties of the vexed question of nomenclature. With the increasing 
knowledge of brain anatomy it is perhaps only natural that 
more exact divisions and distinguishing terms, more in accord- 
ance with the advanced ideas, should be introduced. But 
when familiar terms are used in an altogether unfamiliar sense, 
or applied to entirely different parts of the brain in order to 
give place to new terms no more appropriate than those they 
supersede, the enormous intrinsic difficulty of this subject 
becomes seriously increased, and an undesirable element of 
confusion is introduced. ‘The accurate system of nomenclature 
introduced by His loses much of its value by a confusing 
application of Huxley’s terms ‘ prosencephalon” and “ thala- 
