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insula rises as a round little ball (e) but is totally covered by the 
opercula. 
During the further development, (embryo N°. 11, length 18 ¢.m., 
brainweight 21 grams) it is seen that the part of the sulcus, corre- 
sponding with a in fig. 6 has been pushed more to the bottom and 
lies deeper than the part dc. 
During a still later period of foetal life (embryo N°. 10, length 
19 c.m., brainweight 21 grams) there is not much difference between 
the opercula, corresponding with ab and cd in fig. 6. The only 
thing remarkable is the large prominence of the insula as a small 
round elevation, above its surroundings. It is more distinctly visible 
than one finds in adult monkeys. 
From the above said is proved the great difference in develop- 
ment of the fossa Sylvii in mankind and in semnopithecus. 
In the former the development commences at the base, where 
this is bordered by the lateral edge and moreover in the form of 
a circle or triangle, which later on extends upward, on the lateral 
surface. 
In Semnopithecus one finds exactly the reverse: one sees first 
a suleus on the lateral surface (fig. L and 2), which develops in 
downward direction. Afterwards a second separating sulcus, though 
also on the lateral surface, is added to it (tig. 4 sub c), which too 
develops in downward direction (fig. 5d), by which then only the 
form of a triangle is formed, with its base downward opened. 
A second important difference is the period of complete opercu- 
lisation. 
The length of embryo n°. 12'), in which the fissura Sylvii was 
quite closed, measured 15 cm.-As a new-born Semnopithecus has 
a length of 27 cm., this is a little more than the half of it. The whole 
length of a full term human foetus is + 53 cm., so that a little 
more than its half comes to 29,5 em., which therefore would make 
out the beginning of the sith month. 
As however it is known, that in man the whole operculisation 
only takes place at the end of the 9 month or shortly after birth, 
this is also therefore an important difference. 
A third point of interest is the period in which this sulcus shows 
itself for the first time on the brain surface. 
I pointed out that the first indication of it was found in embryo 
1) The length of the monkey embryo is measured from the middle of the head 
to the root of the tail. The length of a human foetus is taken from head to heel. 
The difference does not alter in any way the above mentioned calculation, as the 
relations remain limited to monkeys reciprocally and also between human foetus. 
