OCR Text |
Show 448 ON CYCLOPIA IN OSSEOUS FISHES. [MiV- l0, bulb and stalk. Such a central cavity would permit the development 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 eyeball, pass along the optic stalk, and form an optic nerve and tracts such as are actually found in type A. The effect 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 primary 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, vitreeus 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 olfactory pits being approximated, the mouth narrowed, and the trabecular 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 m y Trout specimens, being traceable from the cerebral lobes to the small olfactory pits on the under 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 recorded by Paolucci. |