THE BRAIN OF A F@TAL ORNITHORHYNCHUS. 189 
On tracing the cerebrum caudally in a series of sections, the 
basal aspect of the olfactory bulb will be found to be con- 
tinuous with two regions which already present distinct histo- 
logical features, although there is little superficial indication 
of any such division. These two regions are the tuberculum 
olfactorium (figs. 6 and 7, ¢.0.)—a relatively iarge basal 
area next to the middle line—and the pyriform lobe, whose 
surface is covered by the already well-developed ‘ external 
olfactory radiation” (e.0.7r.) of Edinger. The tuberculum 
olfactorium only extends back as far as the lamina terminalis, 
but the pyriform lobe extends the whole length of the hemi- 
sphere (figs. 6—13, pyr. and e.o.r.). In the adult the pyri- 
form lobe is limited laterally by a well-defined fissure (fissura 
rhinalis or ectorhinalis of Turner)—figs. 8 and 14, f.r.,— 
and is separated from the tuberculum olfactorium by a deep 
fissure (fig. 8, f.er.)—fissura endorhinalis of Turner. In 
the foetus there is no sign whatever of the fissura ectorhinalis, 
although a shallow endorhinal groove (fig. 10, f. er.) is already 
to be made out. ‘This appears to correspond to what His has 
wrongly called the rhinal fissure. If this is the case, he has 
excluded from his ‘ lobus olfactorius” the pyriform lobe, 
which does not become superficially distinguished from the 
pallium (Turner) until late in ontogeny. In Notoryctes, 
even in the adult brain, no rhinal fissure appears to be present, 
and in Perameles nasuta it is very shallow and imperfect. 
The rhinal fissure appears late, not only in ontogeny, but also 
in phylogeny. It appears, therefore—and in this I am sup- 
ported by His’s figures—that the so-called rhinal fissure of the 
early human foetus cannot be the ectorhinal—as His believed 
—but the endorhinal fissure. 
The tuberculum olfactorium is not confined to the basal 
aspect of the cerebrum, but crosses the ventro-mesial angle to 
become continuous with the ‘‘precommissural area of the 
mesial cortex ” (figs. 6 and 7, p.a.). In the foetal brain there 
is no line of demarcation between these two regions, since they 
present exactly similar histological features, but in the adult 
(fig. 8 *) there is a clearly defined boundary line. The “ pre- 
