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A pathological process which calls forth the sack (and the latter 
is found in all the cases of this sort) is able to destroy at the 
dorsal brainside the mesencephalon and the cerebellum when they 
hinder its development at the distal end. But at the ventral side 
the pathological process destroys the pedunculi cerebri and the tissue 
on which they lie. This is the germ material out of which jaws, 
mouth and tongue are going to develop. 
Distally bordered by the second branchial arch (the os byoideum 
was intact in all the three cases of synotia) all is destroyed that 
is going to develop out of the first branchial arch (except occasionally 
of the ossicula acustica) and out of the tissue, which lies proximally 
from it. In this way the local defect of the pedunculi cerebri and 
of the tissue forming the middle part of the skull, is easily understood. 
According to the view which I explained in my previous report, 
I think the cause of the sack to be a local process of inflammation, 
which by means of a mechanical influence produces a defect at the 
base of the brain and of the skull. 
On the other hand | acknowledge the possibility that the sack 
and the basal defect, together may be co-effects of another more 
complicated pathological cause. 
In debates upon this subject, held at Leiden Dr. Murk JANSEN 
defended the thesis that the narrowness of the amnion may perhaps 
produce the sack as well as the defect, by compressing the head 
of the embryo in a strictly defined plane. The result may be that 
all the germ material, which is found in this plane, may die. 
Should such a hypothesis be confirmed, there will arise different 
possibilities in the formation of these monstra, but I will not yet 
enter upon these. 
J only wish to lay stress on the foHowing views. 
Destroying of tissue at the proximal end of the skull, so that the 
os ethmoidale disappears and the dorsally placed sack, the roof of 
the distended III ventricle is formed, gives rise to cyclopian monstra. 
But they are not the only defect-formations which are found. 
There is yet another place of predilection, where the tissue that 
will form parts of the skull, may be destroyed. In such cases the 
first branchial arch may be destroyed by pathological processes. 
These lead to synotia. Now and then it occurs isolated. Then we 
see uncomplicated synotia. Nose, eyes and upper jaw are well deve- 
loped, as found in the fetus, mentioned sub I. 
But also the two local destroying processes appear, independent 
of each other, next to one another. Then cyclopian and synotian 
deformities are found together. And there may remain between 
