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Proceedings of Royal Society of Edinburgh. [sess. 
a very early stage of intra-uterine life before the union of the 
fronto-nasal with the maxillary processes. Some cause must have 
been operative which has pushed the fronto-nasal process upwards, 
approximated the two eyes and ocular apertures, fused the two 
optic vesicles, and prevented the development of the pre-sphenoid 
bone and the bones of the nasal septum. In our ignorance of the 
order in which these events took place there must be always a 
difficulty in determining the true cause. Was it, for instance, a 
primary compression of the two eyes towards the middle line, and an 
upward displacement of the fronto-nasal process, or was it a primary 
compression, with fusion, of the two optic vesicles, and a consequent 
directly forward growth of these structures? It will be obvious 
that any cause which directs the optic vesicles forward will prevent 
the development of those cartilaginous and ultimately bony consti- 
tuents of the skull which lie between the two optic foramina and 
in the nasal septum. These are the pre-maxillary bone, the nasal 
process of the superior maxillary, the nasal bones, the lachrymals, 
the ethmoid and vomer — in fact, the very structures which are 
absent from my case. Misformed growth of the optic vesicles will 
also displace the fronto-nasal process upwards, and approximate or 
fuse the two eyes and ocular apertures. An intra-cranial cause, 
therefore, capable of fusing the two optic vesicles together, would 
appear to be able to produce cyclopia. On the other hand, a similar 
condition might be produced by any external pressure capable of 
approximating the two eye-balls. An examination of the cases 
recorded in Dareste’s work on “ The Production of Monstrosities ” 
shows that pressure of the head-fold of the amnion may produce 
this effect. Kundrat has also shown in “ Arhinencephalie ” that the 
result of this will be to press the two optic vesicles more closely 
together, and to drive the cerebro-spinal fluid in a backward 
direction — for the most part into the diverticulum connected with 
the pineal gland, which thus becomes dilated into a large cyst. 
According to Kundrat, there is almost invariably also a dilatation 
of the cavities of the forebrain. There is also sometimes a dropsical 
condition of the third ventricle, with a more or less complete 
atrophy of the optic thalami, and a similarly dilated condition of 
the primary optic vesicle, which remains single ; or, more rarely, the 
two optic thalami may be fused into one. According to Kundrat, 
