THE FISSURA HIPPOCAMPI 167 
demanding immediate and engrossing attention. It is a dis- 
turber of growth rhythms and an obscurer of elementary phy- 
logenetic ‘patterns.’ It is the belief of the writer that there 
is actually some relationship between these two; that is, that the 
acceleration of the neopallium results in a change of rhythm of 
growth in different parts, although it has no effect upon the 
actual differentiation, except that of obscuring it. 
SUMMARY 
1. The medial wall of the cerebral hemisphere of human 
embryos 16 mm. to 30 mm. in length is not ‘perfectly smooth.’ 
Its otherwise even contour is broken by a shallow groove. ‘This 
eroove extends from the region of the olfactory bulb to the 
tip of the temporal pole. It is the fissura hippocampi, the 
‘Bogenfurche’ of His. It is homologous with the fissura arcuata 
of reptiles as described by Herrick. 
2. In embryos as young as 11 mm. the primordial hippo- 
campus can be recognized along the line of the future fissura 
hippocampi. This primordium is identified by the following 
histological peculiarities: 1) a thicker wall; 2) a narrower matrix; 
3) a clearly defined marginal velum; 4) a limiting sulcus, the sul- 
cus limitans hippocampi. This is the first cortical differentiation 
known in man. 
3. The fascia dentata arises in the matrix of the hippocampal 
formation from cells in the dorsal limb of the sulcus limitans 
hippocampi. These cells grow dorsalward, slipping along the 
marginal velum of the hippocampus. In the series studied no 
other cortical differentiation has occurred in this region. It 
is comparable to the persistent primordium hippocampi of am- 
phibians and reptiles. 
4. The fissura prima of His first appears in embryos of about 
25mm. The appearance is coincident with the marked evagina- 
tion of the olfactory bulb. It has no connection with either the 
fissura hippocampi or the hippocampal primordium. 
5. The various regions of the telencephalon medium are 
distinguished by a characteristic morphology and histology in 
all the embryos of this series except the 11.8-mm. In the 
