1906.] CYCLOPIA IX OSSEOUS FISHES. 447 



II. Summary of chief Anatomical Details. 



Olfactory Organs. — Olfactory nerves and pits, reduced in size, 

 are present in all my 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 my 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 i-ecess. 



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 nidimentary 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 trabecule cranii always appear as an absolutely 

 single bar of cartilage underlying the median eye. Either they 

 formed a single structure fi'om the first, and this seems to me most 

 pi-obable, or their fusion was remarkably early and complete. 

 Olfactory capsular cartilages may be present or absent ; when 

 present they ai'e developed in connection with the anteiior 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 remem- 

 bered 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 (fiom whatever 

 cause arising, e. g. partial solidity or coagulation of the jo\k) might 

 bring the optic buds together, and cause them to unite diiring 

 their outgrowth. If only moderate in degree, this pressure might 

 by-and-by allow a central cavity to form in the now single optic 



