THE BRAIN OF. A F@TAL ORNITHORHYNCHUS. 201 
again to form the rudimentary fascia dentata (fig. 15, f. d.). 
Since, in the hippocampus, the cells of the fourth layer of the 
pallium separate from the ependymal layer, the latter appears 
as a much more sharply defined layer (fig. 15). Between the 
ependymal layer and the irregular column of neuroblasts there 
is a relatively wide nerve-fibre layer—the alveus. This is the 
homologue of and is continuous with the fifth layer of the 
pallium. 
This simplicity of hippocampal formation presents a close 
resemblance to the corresponding region in the reptile brain. 
The anterior extremity of the hippocampus in all non- placental 
mammals maintains this primitive simplicity of structure into 
adult life (fig. 8). The much-disputed questions of homology 
will be discussed later on in dealing with the histology of the 
adult organ. 
The fornix system is as yet imperfectly formed, and presents 
a reptilian-like simplicity of arrangement. Although the de- 
scending fibres of the fornix (figs. 7,9, 15, d.f.) are already 
well formed, there are only a few scattered commissural fibres 
in the lamina infra-neuroporica. 
The Pyritorm Lobe: 
As the pallium is traced into the pyriform lobe (figs. 6, 7, 
9—12) the molecular layer suddenly increases in thickness. 
This layer contains the fibres of the external olfactory radia- 
tion (e. 0. 7.).. The superficial nerve-cell layer suddenly 
becomes more scattered, and loses the regular appearance 
_ found in the pallium proper. This layer corresponds to the 
layer of “double pyramids” (Kolliker) found in the adult 
pyriform lobe (fig. 8). The other layers of the pallium are 
represented in the pyriform lobe merely by diffusely scattered 
nerve-cells. Behind the lamina terminalis the pyriform lobe 
is continuous mesially with a large mass of diffusely scattered 
nerve-cells—the nucleus amy gdale. 
The Corpus Striatum and Optic Thalamus. 
The corpus striatum (c.s.) forms a large mass projecting 
into the lateral aspect of the lateral ventricle, and lying in the 
