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Show 1903.] MOXSTROSITIES IX FISHES. 21 surrounding the fused adjacent Meckelian and hyoid bars in Class I. (p. 10) are destitute of nerves. Development of parts may take place without the presence of nerves of general sensation. Thus in Class I. {a) no trace exists of adjacent trigeminal nerve-fibres, yet the inner sides of the snout and head are well developed wherever they have sufficient space. On the whole, the anatomy of the series of monstrosities which has been described in this paper is remarkably symmetrical, both twins contributing equally to the sum of structures in the transitional region, in which, moreover, the law that union takes place between homologous structures alway holds good. EXPLANATION OP THE PLATES. (Reconstruction Diagrams.) PLATE I. Fig. 1. Cranial and hyomandibular skeleton of a normal Trout embryo, seen above, after removal of the roof-cartilages by a section supposed to pass horizontally through the nasal and periotic cartilages (pp. 5, 6). a, olfactory pit and cartilage. b, trabecule cranii. c, parachordal cartilages. d, placed just in front of pituitary space. e, „ behind pituitary space. f, palato-quadrate bars. g, Meckel's cartilage. h, hyoid bar. i, hyomandibular. k, placed in floor ot periotic capsule. in, pituitaiy space with external recti muscles passing through it. n, notochord embedded in the parachordals. Fig. 2. Roof-cartilages of skull of a normal Trout embryo, seen from above (p. 6). (For lettering see under fig. 6.) Fig. 3. Cranial and hyomandibular skeleton of a double-monster embryo, seen from above as in tig. 1. The duplicity affects the anterior cranial structures only (pp. 7, 8). Lettering as above ; see also under fig. 5. Fig. 4. Roof-cartilages of skull of the double monster illustrated in fig. 3. For lettering see under fig. 6. Fig. 5. Cranial and hyomandibular skeleton of a double monster exhibiting a slightly greater degree of duplicity than that of tig. 3 (pp. 11, 12). In figs. 3 & 5, a, b, c, &c. represent the same structures as in fig. 1; f, g', i', k' are the inner or adjacent (reduced) structures corresponding to the fully-developed outer f g, i, k (i. e. palato-quadrate, Meckelian, hyomandibular, and periotic cartilages). Fig. 6. Roof-cartilages of skull of the double monster illustrated in fig. 6. f\, f2, f3. Anterior, middle and posterior fontanelles. r, cartilage of roof of nasal pits. s, supraorbital bars («', adjacent reduced pair). t, cartilage of roof of periotic capsule. w, parachordal cartilage. x, y, tegminal cartilages above third ventricle and cerebellum respectively. Fig. 7. Vertebral cartilages in normal Trout embryo (p. 6). fio-, 8. „ „ )> any double monstrosity near the transitional region. Spc. Spinal cord. Nch. Notochord. Na. Neural arch cartilage {Na', adjacent reduced neural arch cartilage). Ha. Haemal arch cartilage (Ha', adjacent reduced haemal arch cartilage). |