THE FISSURA HIPPOCAMPI 159 
During this time of development a new groove appears on the 
medial wall, the result of active olfactory bulb evagination, the 
fissura prima of His. The 27.8-mm. and the 32.1-mm. belong to 
this group. In the 39.1-mm. and the 43-mm. the anterior seg- 
ment of the fissura hippocampi has disappeared, but the posterior 
and the fissura prima persist. The cortical lamination of the 
dorsal lip of the fissura hippocampi is coincident with the flatten- 
ing of the medial wall. 
If a dorsal commissure were added to the anterior commissure, 
now lying in the much-thickened lamina terminalis, the rela- 
tionships of commissure, fissura hippocampi, and fascia dentata 
would resemble those of the marsupial. Elliot Smith (97, p. 
67) wrote of this comparison as follows: 
In the Marsupial we have a fissura arcuata or hippocampi, extending 
from the tip of the temporal pole right round the mesial wall of the 
hemisphere towards the olfactory peduncle; so, in the fetal child or 
kitten, we find the Bogenfurche (which we might, with Mihalkovics, 
appropriately call ‘Ammonsfurche’) following a similar course and 
shading away towards the cephalic pole of the hemisphere. And it 
is necessary to remark, in passing, that the so-called part of the ‘Vor- 
dere Bogenfurche,’ which His calls ‘fissura prima’ has nothing whatever 
to do with the true Bogenfurche or fissura arcuata, if we regard the 
latter as the primitive fissura hippocampi. 
Smith refers to the 1891 paper of Marchand. Moreover, 
Marchand (’09) denied the existence of such a fissure and Smith 
(03) reports that Hochstetter’s work on fissuration of the medial 
wall proves beyond a doubt that all the fissures are artefacts. 
But the conditions of these tissues in the brains of the 39.1-mm. 
and the 43-mm. are essentially the same as described by Smith 
in his first paper. His himself (’04) states plainly that the fissura 
prima has no relation to the fissura arcuata or the hinterer 
Bogenfurche; he defines it as follows: ‘‘The continuation of the 
fissura mesorhinica extends for a distance over upon the medial 
wall of the hemisphere as the fissura prima. By a deepening 
of the surrounding sulcus the termination of the lobus olfactorius 
or this bulbous portion becomes separated more and more from 
the overhanging frontal lobe. The bulbous portion retains 
its sagittal direction and becomes separated laterally from a 
