183 
and polymorphous cells of the cornu Ammonis become continuous 
at the upper (figs. 6 and 8) border of the fascia dentata with the 
stratum granulosum and nucleus fascie dentate. The fascia 
dentata, together with the nucleus fasciz dentate, forms a single 
convolution, so that the opinions of Duval (7), Golgi, and Sala 
(32), that these two parts are independent receives no confirma- 
tion from the examination of the Metatherian condition. The 
fascia dentata (including the nucleus fascize dentate, as Henle, 
Krause, Huguenin, Schwalbe, Meynert, and Obersteiner, among 
many others, consider it) forms the edge of the true cortical 
structure, although it is nowhere a morphologically free edge. In 
the greater part of its extent the hemisphere wall has a morpho- 
logical representative in the epithelial fold of the choroid plexus. 
In front of the foramen of Monro, where the choroid fissure is 
absent, the fascia dentata (with its deeper layer, the “ nucleus ”) 
becomes continuous inferiorly with another part of the hemi- 
sphere wall, which lacks a cortical structure—the precommissural 
area, which Meyer has described as a ganglion mass without 
cortex (26). Thus the fascia dentata in the whole of its extent 
forms the edge of the true cortex. These statements are borne 
out by the examination of series of fetal Ornithorhynchus, 
Perameles, and Macropus brains. 
Although it will be granted that the pes hippocampi is a very 
specially modified part of the cortex, still there does not appear 
to be any justification for believing, with Giacomini, that it does 
not represent any convolutions, since it is already highly con- 
voluted in brains otherwise smooth. The hippocampus is equally 
convoluted in all Mammals, because it reaches its maximum 
development quite early in the phylogenetic history of the indi- 
vidual. Thus in Platypus it possesses a histological differentia- 
tion, quite as complex and fine, as is found in the highest mam- 
mal. Like the pyriform, it is developed early both in phylogeny 
and ontogeny in accordance with the development of the olfactory 
apparatus. Because part of the smell centre should reach a high 
state of development, when the pallium is not proportionately 
intricate, is no argument that the cortex of the smell centre does 
not behave like the rest of the cortex in similar circumstances. 
It should be noted, however, that, intimate as is the connection 
between the hippocampus and olfactory lobe, the relative sizes of 
the two parts are by no means constant. Thus, in spite of the 
marked relative difference in the sizes of the olfactory bulb in 
Ornithorhynchus and Perameles, there is little appreciable differ- 
ence in the sizes of their hippocampi. In Wotoryctes the size of 
the hippocampus is relatively small, considering its huge olfactory. 
What determines the size of the hippocampus is hard to say. 
In studying such a brain as that under consideration, one can- 
