941 
suleus therefore, is not much more than a hook, of which the 
longest side is formed by ab, and the shortest by c. 
It is remarkable however, that whereas the top part 6 of the 
posterior wall of the fissura Sylvii is not deep, this on the other 
hand is the case with the part c. This would give the impres- 
sion as if this latter part corresponds with h in fig. 2. Against 
this would plead that one sees in fig. 1 as well as in fig. 2 the 
existing suleus 5 connected by a superficial curvation with the lateral 
edge, what must be the sulcus a in fig. 4. Yet the first view 
remains possible, if one accepts, that during the following develop- 
ment the fold a (fig. 2) is continued upward from the corner in 5 
and that it uplifts the latter suleus as it were at its posterior end, 
moving it also upward. 
The point of junction between a and 5 (tig. 2) in that case would 
be the same as between b and e (fig. 4), with this difference, that 
it is removed in postero-superior direction. When in the last figure « 
is the oldest part and 5 the youngest, then it is comprehensible that 
a is deeper than 6. This conception would be supported by the 
fact, that the direction of 6 in fig. 1 is a more perpendicular one 
than in fig. 2, so that the position of this fold in these three 
periods of development presents itself as in fig. 1 and 2, and c 
in fig. 4. 
As however the link is missing between fig. 2 and fig. 4, which 
might solve this riddle, we must be contented to conclude, that: 
in fig. 2 the part 6 is deeper than the junction a with the 
lateral edge; 
in fig. 4 the reverse is found, hence a is deeper than 6 and c 
too is deeper than 6. 
In embryo 135, probably of a more advanced growth than 18a 
we find the anterior wall of the fissura Sylvii totally formed (fig. 5), 
although the border in d may not yet be strongly developed. The 
image which is now formed, totally agrees with that which is also 
found in human embryos. (Rurzius plate I fig. 33 and 35, plate III 
fig. 3) i.e. the form of the triangle with the top upward and the 
opened base directed downward. 
In. embryo N°. 12 (length 15 em. and brainweight 12 grams) the 
fossa Sylvii is completely closed and the insula is operculised. It 
one opens the walls of the sulcus (fig. 6) then it appears that the 
image, given in fig. 5, in large features has remained. The oper- 
culum temporale is still most developed, much more than the 
operculum frontale in d. On the other hand it is seen, contrary to 
tig. 5, that the junction dc has sunken into the depth, while the 
