OCR Text |
Show 1903.] MONSTROSITIES IN FISHES. 5 Trout embiyos at a corresponding age-three weeks after hatching- is also given. The frequency of occurrence, the causation, the general appearances, and the classification of double monstrosities in fishes have been the subject of many papers and notices, lists of which are given by Prof. B. C. A. Windle in the ' Proceedings' of the Zoological Society of London, 1895, p. 423, and by Dr. Franz Schmitt in the ' Archiv fur Entwicklungsmechanik der Organ-ismen,' Bd. xiii. p. 34. The classification adopted here is on the same lines as that of Windle {I. c), and has special reference to the material at my disposal. All m y specimens were examples either {a) of union by the yolk-sac, or (6) of anterior duplicity. The former require only to be mentioned here, as each of the twin bodies contains a complete and separate complement of organs, while the latter may be conveniently divided into- Class I. Union in head-region : {a) the twin brains united at the optic lobes; {b) the twin brains united at the medulla oblongata. Class II. Union in pectoral region : (a) the adjacent pectoral fins not represented ; {b) the adjacent pectoral fins present, but united and reduced in size. Class III. Union at posterior part of body : {a) the two alimentary canals united a considerable distance in front of the vent; {b) the two alimentary canals united close to the vent. ANATOMY OF NORMAL TROUT EMBRYOS. In normal Trout embryos of the same age as the monstrosities, ossification has not yet begun. The parachordal cartilages are uniting round the anterior end of the notochord and have already joined with the trabecular cranii, which, coalescing in front of the pituitary space, run forwards as a median flattened bar to meet the nasal cartilages. The pituitary space gives passage to the choroidal and internal carotid arteries and to the back part of the musculus rectus oculi externus (PI. I. fig..l). On either side, the parachordals have grown upwards in the form of laminar plates, which meet in the mid-dorsal line over the upper part of the medulla, but leave a narrow V-shaped fontanelle over the lower part (PI. I. fig. 2, f 3). The auditory capsules are firm bosses of cartilage, moulded on the labyrinth, closed externally, but widely open towards the brain. Dorsally, they are connected with each other by a thin vault of cartilage roofing the cerebellum ; anteriorly, they are continuous with the supraorbital bars to be afterwards described; ventrally, they join the trabecular and parachordal cartilages; and externally they articulate with the hyomandi- |