THE BRAIN OF A FQ@ITAL ORNITHORHYNCHUS. 197 
(fig. 8) to draw any line of demarcation between corpus 
striatum and tuberculum olfactorium. This being the case, 
this region (fig. 11, *) may doubtfully be homologised with 
the corpus striatum, as His has done. It will be noted from 
the figures that the transition from undoubted hypothalamus 
to this doubtful region takes place at the optic chiasma—the 
termination of His’s ‘‘ Grenzfurche.” If, therefore, it is granted 
that this doubtful area (*) is corpus striatum, it follows that 
the hemisphere brain extends down as far as the optic recess, 
as His has described in the case of the human feetus. 
Having now given a general account of the disposition of 
parts, there are certain regions which need further consideration. 
The Olfactory Bulb. 
This part of the cerebrum, whose structure is roughly re- 
presented in transverse section in fig. 18, consists of four 
distinct layers, as Vignal! has pointed out. Immediately sur- 
rounding the ventricle there is a thick mass of small cells with 
deeply stained nuclei. This is the ependymal layer (e). 
Surrounding it there is a stratum of about the same thickness, 
but much lighter in appearance—the layer of nerve-cells 
(6). Examined with the high power, this is found to consist 
of amass of well-formed neuroblasts (fig. 19). Posteriorly this 
layer is found to be continuous with the essentially nervous part 
of the cerebral cortex. It is probable that in the adult the 
neuroblasts which compose this layer mostly develop fibres of 
the pars olfactoria of the anterior commissure. The most 
superficial layer is imperfect, consisting of two distinct gan- 
glionic masses, each composed of neuroblasts like the second 
layer. The larger mass (olf. g.) covers the whole inner surface 
of the bulb, and extends on to the ventral and slightly on to 
the dorsal aspects of the bulb. In this region the true olfactory 
nerves terminate. Hence I have distinguished it as the 
“olfactory” ganglion. On the outer aspect of the bulb there 
is a second smaller mass (J. g.), presenting’ exactly similar 
histological features. In this mass the fibres derived from 
1 Quoted in Minot’s ‘Human Embryology.’ 
