THE BRAIN OF A F@TAL ORNITHORHYNCHUS. 191 
hippocampal or fornix commissure. In the adult Platypus 
there is no difficulty whatever in recognising the hippocampus, 
since its histological features, whether studied by aniline blue- 
black staining or by the methods of Weigert or Golgi, exactly 
resemble those of the higher mammal (figs. 8 and 14). Upon 
this fact largely depends the extreme morphological import- 
ance of the cerebrum of Ornithorhynchus. For while the 
topographical relationship of the Prototherian hippocampus 
resembles that of the Sauropsida, it at the same time presents 
quite as high a histological differentiation as is found in the 
Eutheria—a fact which renders its identification quite certain 
and easy. By comparison it is easy to identify the correspond- 
ing region in the foetal brain, where the simplicity of its 
histological structure (fig. 15) and the absence of the com- 
plicated foldings and inrolling of the adult structure vividly 
recall the Sauropsidan hippocampus, and at a glance convinces 
one of the homology. In all adult Mammalia the pallium 
takes some share in the formation of the mesial cortex (figs. 8 
and 14, p.), but in the foetus the hippocampus extends right 
up to the supero-mesial angle, so as to exclude the pallium from 
any share in the formation of the mesial wall. Similarly in 
the reptile the hippocampus forms the greater part of the 
mesial wall, and extends forwards above the commissures tc- 
wards the anterior pole of the cerebrum, as Edinger and 
Meyer have described. Not only does it form the mesial wall, 
but it extends on to the dorsum of the cerebrum. Herrick, 
by attempting to draw too close a comparison with the 
Eutherian brain (where this region is greatly disturbed by the 
development of a corpus callosum), would limit the hippo- 
campus to the posterior extremity of the hemisphere. This 
is a gratuitous assumption, utterly unsupported by any evidence. 
The whole of the dorsal and lateral aspects of the surface 
of the hemisphere, from the hippocampus above to the pyriform 
below, constitutes the pallium (Turner). Both in its onto- 
genetic and phylogenetic history it continues to increase in 
extent after the more ancestral hippocampal and pyriform 
regions have reached their full development, both in size and 
