44 CYCLOPIA— CAUSATION, ETC. 



stage, this fusion being more or less complete in the different embryos. He thought it probable 

 that the resulting large optic cup induced the formation of a single lens, the lens being formed of 

 ectoderm different in position from that of the normal lens-forming cells. He adds that probably 

 there is no localisation of lens-forming substance in the ectoderm of the fish embryo, and that it is 

 the underlying optic cup which induces actual lens formation on the part of the ectoderm. This is 

 more or less in harmony with the result of recent experimental work on the development of the 

 amphibian eye. On the other hand, it would render difficult of explanation those cases of defective 

 heads in embryo fish which possess one or more lenses without any trace of corresponding optic 

 cups. 



Semicyclopia and allied conditions. Alike in single embryos and in double monsters, one may 

 come across examples in which the heads show lateral compression. In extreme instances, the 

 whole anterior part of the head may be atrophied, the mouth being deficient, the brain pro- 

 foundly malformed, and the eyes absent or represented only by the lens. (See PL XXV. fig. 106, 

 XXVII. fig. 112.) When the defective condition is moderate in degree, the eyes and the olfactory 

 pits are brought together, the mouth narrowed, and the trabeculae cranii ventrally displaced, so that 

 they lie below the approximated eyes and fail to join the olfactory cartilage in front. This 

 condition, which may be called semi-eyclopia, is illustrated in PL I. fig. 3 and XXVI. figs. 109, 

 110. Similar specimens are figured by Girdwoyn (SI p. 112) and by De Quatrefages (198 

 PL VII. fig. 2). See also p. 51. 



Causation. 1. It seems probable that the causes which produce cyclopia in fishes are 

 sometimes purely mechanical, and that under favourable circumstances they may exercise a minimal 

 degree of interference with the development of other structures, in particular with that of the brain. 

 Under ordinary circumstances, and apart from such experiments as those of Stockard, pressure, or at 

 any rate want of space for development, may be put down as the commonest causal factor. The 

 egg-membrane of the trout is tough and strong, while the cavities of the optic bulb and stalk, and 

 even that of the central nervous system, are developed secondarily in solid masses of cells. It may 

 be supposed that undue lateral pressure, from whatever cause (e.g. partial coagulation of the yolk, 

 swelling due to differences of osmotic pressure, want of space from twinning of the head), may bring 

 the optic buds together and cause them to unite during their actual outgrowth. If only moderate in 

 degree this pressure might next allow a central cavity to form in the single 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 may greatly hinder 

 the formation of a central cavity in the primary optic vesicle and stalk. This might prevent 

 the formation of a choroidal fissure by the usual method of ventral cupping. In the absence 

 of a choroidal fissure, mesenchyme 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, without choroidal fissure, vitreous humour, or optic nerve. 



2. The apparently specific action of magnesium chloride in producing cyclopia is at present 

 impossible of explanation if, as Stockard seems to have proved, it does not depend on differences of 

 osmotic pressure. Stockard (:237), however, points out that probably other substances are capable of 

 exercising other specific effects on developing eggs, and calls attention to the characteristic Lithium 

 larvae of Herbst, to Morgan's frog larvae and to his own fish embryos. 



3. Although positive evidence is a wan ting, I think it extremely likely that cyclopia may 

 sometimes have a spontaneous or autogenetic origin. In this respect it would resemble various 



