= 198 
losum on the other, press upon the hippocampus proper, which is it- 
self in a state of active growth. As a result the whole thickness of 
the ventricular wall becomes bulged in producing in the ventricle the 
swelling of the pes hippocampi and on the surface the Fissura hippo- 
campi. This stage is admirably demonstrated in a transverse section 
of the anterior extremity of the hippocampus in Ornithorhynchus 
(Fig. 2). The region of the Fascia dentata occupies the same relative 
position as it does in the reptile (compare Fig. 1), but the hippocampus 
proper takes a large ventricularly directed curve. The molecular layer 
in the region of this curve becomes greatly broadened from the fact 
that the layer of pyramidal cells recedes from the surface. It will 
be further noticed that the Stratum granulosum is directly continuous 
with the layer of pyramids. Concerning this GoLG1 says!) “che la 
Fascia dentata sia una diritta continuazione, anzi un’ espansione della 
lamina grigia circonvoluta”. 
As the hippocampal formation is traced backwards in a series of 
transverse sections of the adult Platypus, it will be found (Fig. 3) that 
the Stratum granulosum (together with its molecular layer) rapidly in- 
creases in breadth, so that its continuity with the layer of pyramids 
becomes disturbed, and the two cellular lamellae slightly overlap. 
This breach of continuity has been the cause of much of the per- 
plexity, which surrounds this region in higher mammals. Brit ?) 
would appear to regard it as an essential feature of the hippocampal 
formation not only in mammals, but also in reptiles. But, as Figure 2 
clearly shows, the break is wanting in parts of the hippocampus of 
certain mammals. In the reptile (Pseudechis, Iguana, La- 
certa) I have completely failed to find the break of which Britt 
speaks. In fact I should have been surprised had I found it, since 
its causal factors are wanting in the reptile. ApoLF MEYER?) agrees 
with me in denying its presence in a number of reptiles. 
It will be noticed that the layer of polymorphous cells does not 
increase pari passu with the Stratum granulosum. In other words 
the essential factor in the morphogenesis of this re- 
gion is a localised hypertrophy of the superficial layers 
of the cortex, which leaves the deepest part (near the ventricle) 
unaffected. 
As one examines the series of transverse sections it will be found 
1) loc. cit. 
2) loc. cit. 
3) loc. cit. 
