( 204 ) 
of the fossa gives rise to the formation of the operculum triangulare 
in most human brains. On the other hand the separation of the insula 
surface from the frontal cortex by the sulcus opercularis in human 
embryos is already complete long before the opereulisation is achieved. 
From this it results that there are two differences between the 
development of this region in phylogenetic and ontogenetic sense. 
1. The confluence of the sulcus opercul. with the sulc. fronto-orb. 
which phylogeaetically is the condition of the total opereulisation of 
the frontal cortex and of the formation of the rr. ant. foss. Sylvii, 
apparently is not required in embryologic development, the sulc. fr. orb. 
wanting in human foetus. 
2. An entire separation of the insula from the adnexa is not 
necessarily followed by total opereulisation. The latter is demonstrated 
on the left hemisphere of our case where the fronto-orbital cortex 
is clearly separated from the insula surface by a rather deep furrow. 
Looking for an homology with the rr. anteriores one might be 
inclined to consider the furrows a and as such. Topographically 
the fact that they seem to limit the operc. triangulare might confirm 
this supposition. 
If, however, we consider the structure of the right hemisphere, 
it appears that this operculum clearly shows a pli de passage between 
insula and frontal cortex (figure 3 f ). Considering the way by 
which the rr. ant. and the operc. triang. are formed in the foetus 
we may exclude the possibility that a part of a cortex that has 
already been transformed into operculum, should become again 
immediately continuous with the insula simply by a cortex-bridge. 
It seems to be more probable that the caudal expansion of the 
frontal cortex in this abnormally developed brain was especially m 
the right hemisphere so irregular, that instead of one, two separated 
sulci operculares were formed, which do not meet in the later develop¬ 
ment. On the right side the opercular sulcus, necessary for the for¬ 
mation of an operculum orbitale does not occur. Both insulae show 
the particularity that they cohere directly with the region behind t e 
fiss. Rolando by a large circumvolution; only the frontal part oft e 
operculum Rolandicum is partly formed. 
The question is, to ivhich of the surrounding surfaces, that usw y 
take part in covering the insula , the unsufficient opereulisation ts 
due. Where the latter has not developed at all, the answer can 
easily be given. 
Certainly the retrorolandic cortex has failed in growth. This 
does not explain the uncovered condition of the ventral part o 
insula. 
