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Show 1906.] ABNORMAL TROUT EMBRYOS. 451 right side of the right twin. This gives an even more complex mixture of " individualities " than is found in ordinary cases of symmetrical double monstrosity. In this case of aborted twin head the lens alone of all the eye-structures has survived. This is by no means infrequent, even in cases of atrophy of the head uncomplicated by duplicity. A good example is shown in fig. 3, which illustrates a transverse section of a single atrophic head. The mouth, the lower jaw, the trabecular, and the palato-quadrates are absent. One large lens, clothed with muscle-fibres, is present on the right side ventrally and compresses the lower part of the brain. A second smaller hour glass-shaped lens lies beside it, all other ocular structures being deficient (Plate X X X I I I . fig. 3). (B) Local Deficiency or Reduplication of the Notochord in Trout Embryos. While examining a number of Trout embryos in serial section I came across three cases of local reduplication of the notochord. In two of them the notochord is bifid at its anterior extremity, becoming single while still in the intra-cranial region. The parachordal cartilages are broad in front and enclose both ends of the notochord. There is no duplicity of any other structure. It is perhaps remarkable that one of these embryos was a cyclops of type B. The third example of reduplication of the notochord was found in a set of sections which had been cut from an apparently normal embryo for the purpose of serving as a typical series. In the middle abdominal region the notochord is observed to divide into two limbs which lie adjacent to, but quite separate from, one another for four or five segments, and then unite again. Where they are widest apart each has a separate sheath and separate sets of neural and hsemal arch cartilages. The adjacent cartilages are disposed, exactly as in double monstrosities, at the region of transition from the double to the single condition. These cases seem to be examples of local fission affecting a single axial organ, rather than examples of true axial duplicity. Local deficiency of the notochord occurred in one specimen. Here the notochord, which is normal in the cranial and cervical regions, ceases abruptly just behind the level of the pectoral fins. After being absent for six somites, it reappears and runs backwards normally along the rest of the trunk. Plate X X X I I I . fig. 4 illustrates the appearance of a transverse section in the defective region. The neural and harmal arch cartilages have fused together to form a series of half-rings below the cord. Ventral to these the lateral muscle-masses meet one another in a mesial raphe above the dorsal aorta, forming a strong support and sling for the vertebral column and the cord. PROC. ZOOL. Soc-1906, VOL. I. No. X X X . 30 |