OCR Text |
Show 1906.] CYCLOPIA IN OSSEOUS FISHES. 447 II. SUMMARY OF CHIEF ANATOMICAL DETAILS. Olfactory Organs.-Olfactory nerves and pits, reduced in size, are present in all m y specimens. The olfactory pits lie close together on the inferior aspect of the mesial frontal process. Brain.-Fusion of the posterior parts of the cerebral lobes is found in all m y specimens. By itself, as in type A, this condition is compatible with the presence of a well-developed cyclopean eye possessing vitreous humour and an optic nerve, as well as with the presence of pineal diverticula, hypophysis, and hypoaria, and of optic tracts and optic recess. Fusion of the basal structures in the mid-brain, as in type B, is associated with greater defects-viz., reduction in the size of the eyeball, absence of choroidal fissure, optic nerve and optic tracts, and absence or rudimentary condition of hypophysis and hypoaria. Dropsy of the central cavity of the brain is conspicuous by its absence. Eye.-As seen in type A, the eye may be remarkably well-developed, possessing lens, retina, vitreous humour, retinal pigment, and optic nerve. A double set of normal eye-muscles, excepting only the internal recti, may be present. Paired superior and external recti are constant, while the superior obliqui and the inferior recti are variable. The remarkable set of conditions which accompanies fusion of mid-brain structures has been mentioned above in connection with the brain. Skeleton.-The trabecular cranii always appear as an absolutely single bar of cartilage underlying the median eye. Either they formed a single structure from the first, and this seems to m e most probable, or their fusion was remarkably early and complete. Olfactory capsular cartilages may be present or absent; when present they are developed in connection with the anterior ends of the supra-orbital bars. The palato-quadrate, the mandibular and the hyoid bars tend to be shortened, in correspondence with the general transverse narrowing of the mouth-parts. III. CAUSATION. My specimens are not young enough to afford direct evidence regarding the mode of origin of the cyclopic condition. Probably pressure is the causal factor in most instances. It will be remembered that the egg-membrane of the Trout is tough and strong, and that the cavities of the optic bulb and stalk and even of the central nervous system are developed -secondarily in solid masses of cells. It may be supposed that undue lateral pressure (from whatever cause arising, e. g. partial solidity or coagulation of the yolk) might bring the optic buds together, and cause them to unite during their outgrowth. If only moderate in degree, this pressure might by-and-by allow a central cavity to form in the now single optic |