448 ON CYCLOPIA IN OSSEOUS FISHES. [Apr. 10, 



bulb and stalk. Such a central cavity would permit the develop- 

 ment of the secondary optic vesicle with its choroidal fissure. The 

 choroidal fissure would enable mesenchymal cells to pass into the 

 interior of the eyeball and form a vitreous body, and would enable 

 also nerve-fibres growing from the retina to escape from the eye- 

 ball, pass along the optic stalk, and form an optic nerve and tracts 

 such as are actually found in type A. The efifect of moderate 

 pressure on the brain may perhaps be recognised in the fusion of 

 the posterior parts of the cerebral lobes characteristic of this same 

 type. 



A greater amount of lateral pressure might lead to such further 

 degrees of fusion affecting the third ventricle and the mid-brain 

 as are illustrated in type B. In the eye it might greatly hinder 

 the formation of a central cavity in the piimary optic vesicle and 

 stalk. This condition might prevent the formation of a choroidal 

 fissure by the usual method of ventral cupping. In the absence of 

 a choroidal fissure, mesenchyma could not enter behind the lens to 

 form a vitreous humour, and nerve-fibres formed in the retina 

 would have no exit from eyeball to stalk, and the stalk itself 

 would degenerate. The condition in type B might then be realised, 

 i. e. an eye, reduced in size, with choroidal fissure, vitreous humour, 

 or optic nerve. 



Analogous conditions, almost certainly due to pressure, are 

 sometimes seen in double Trout monstrosities. One or both of the 

 twin heads may show lateral compression, the eyes and the olfactoiy 

 pits being approximated, the mouth narrowed, and the trabeculse 

 cranii ventrally displaced. In extreme cases the whole anterior 

 part of the head may be atrophied, the mouth being deficient, the 

 brain profoundly malformed, and eyes absent or represented only 

 by a lens. 



IV. Comparison with Cyclopia in Mammals. 



1. While olfactory nerves do not seem to have been demonstrated 

 in any mammalian cyclops, they are present in all my Trout speci- 

 mens, being traceable fi'om the cerebial lobes to the small olfactory 

 pits on the undei- surface of the frontal process. If, as seems certain, 

 this process represents the " proboscis " of a cyclopean mammal, 

 the " proboscis " can have no relation with parts of the brain 

 behind the cerebral lobes and in particular none with the 

 hypophysis. 



2. Dropsy of the central cavity of the brain is not characteristic 

 of cyclopia in fishes. This may be contrasted with the usually 

 saccular condition of the cerebral lobes in cyclopean mammals. 



3. The relatively good development of all parts of the brain, 

 particularly in type A, is remarkable. Indeed there seems to be no 

 reason why a specimen of this kind should not be able to survive 

 and obtain food for itself, as in the apparently unique case recoi-ded 

 by Paolucci. 



