THE BRAIN OF A FATAL ORNITHORHYNCHUS. 185 
mencephalon ” in quite a different sense from that in which they 
are universally used, which must prevent the general acceptance 
of his suggestions. Although His’ system of nomenclature is 
otherwise very convenient for descriptive purposes, the mor- 
phological value of his divisions is seriously open to question, 
especially the somewhat arbitrary limits to his “ Endhirn.” 
The nomenclature of v. Kupffer seriously adds to the confusion. 
One cannot but admire the exactness of Burt Wilder’s definitive 
terms, but they are so uncouth and bizarre that the English 
student has to learn a new language before he can master the 
simplest description. In this paper the most unambiguous 
terms will be selected from the different systems, the alterna- 
tive names being indicated in brackets; but as far as possible 
Huxley’s well-known nomenclature will be employed. 
Hach cerebral hemisphere consists of an ovate mass, attached 
to the lateral aspect of the front end of the neural tube. The 
maximum length of each hemisphere, including the bulbus 
olfactoril, is 5°15 mm., and at its mid-point its height and 
depth are about half that measurement. The diencephalon 
(with the median part of the “ Endhirn”), whose extent is 
schematically represented by the shaded area in fig. 1, is of 
the same depth as the posterior part of the hemisphere, but is 
much shorter. Posteriorly the hemisphere overlaps the me- 
sencephalon (mes.) to a slight extent. 
In a median sagittal section through the fore-brain, which 
is represented somewhat diagrammatically in fig. 2, the optic 
nerve is seen as a prominent object (opt. n.) in the floor, so 
that it forms a convenient starting-point. Immediately behind 
the optic nerve the floor sinks to form the recessus infun- 
dibuli (7. i.) of His (recessus postopticus of Burckhardt), 
from the posterior extremity of which the tubular hypophysis 
(hyp.) extends at first downwards, and then bends forwards at 
its extremity. ‘The anterior wall of this extremity contains a 
number of neuroblasts and a distinct layer of nerve-fibres, 
which extends upwards on to the floor of the third ventricle. 
The bent extremity of the tubular hypophysis is closely sur- 
rounded by the large glandular (buccal) part of the hypo- 
