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Show 428 DR. B. C. A. WINDLE ON DOUBLE [May 21, formation of the jaws, as in the following cases:-(a) In this case the smaller head was unprovided with eyes and the lower jaw was very short and tapered rapidly to a sharp point. (6) In this case the upper jaw was much shorter than the lower, which projected a long way in front of it: there was, in fact, an almost complete failure to develop of that portion of the head which is placed in front of the eyes, (c) In this case the condition was carried still further, for so extensive was the arrest of the development of the anterior part of the skull that the two eyes were placed quite close to one another, with scarcely any separation between them. These two last forms approximate to the condition known as cyclopia. 3. Frequency of occurrence. Jacobi, as has already been mentioned, stated that double monstrosities occurred more frequently in some years than in others ; and I remember the late Mr. Burgess, the founder of the fish-hatching establishment at Malvern, making exactly the same remark to me, though he was unable to assign any cause for the variation. They have been met with amongst Sharks, Mackerel (15), Salmon, Trout, Perch, and Pike. I do not know whether any statistics have been drawn up at any of the fish-hatching establishments as to the percentage of double monsters to normal forms, but the following figures have been obtained from data in some of the papers to which I have referred:- Observer. Fish. No. Examined. No. of Double monsters. Bauber. Trout. 1000 2 Pike. 325 1 Coste (16). Various. 400,000 Over 100 Lereboullet. Pike. 203,962 222 From the last figures, which are the most complete, it would appear that the percentage of occurrence in the pike is rather less than "1, or about one per thousand. With regard to the frequency of occurrence of the various classes into which I have divided these forms above, the distribution in the cases which I have examined myself is as follows:- 1. Three eyes, all one size 3 2. „ median being larger than others 1 3. Four eyes 1 4. T w o heads 0 5. Fission extending to the pectoral region 11 6. „ extending to posterior border of yolk-sac. ... 7 7. „ extending beyond posterior border of yolk-sac 10 8. Tails overlapping 9 9. Union only by tails 0 10. Anakatadidymus 2 11. Parasitic |
