分割前に遠心分離處理を施したアズマヒキガエルの卵から發生する二重複胚と三重複胚
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By centrifuging the eggs of a toad, Bufo vulgaris formosus, before the first cleavage, Motomura (1935, '49) obtained double tailed monsters. Since these monsters are caused by the occurrence of secondary invagination at the animal pole in addition to the primary one in the vegetal hemisphere, they are quite different in type from that of the twins hitherto reported. Okada and his coworkers (1947, '49) could also obtain double headed monsters and double tailed monsters by centrifuging toad's eggs, and oil drops that are accumulated at the centripetal were assumed to be the cause of these abnormalities. To obtain further informations, I have repeated a similar kind of experiment in the spring of 1948. Toad's eggs before the first cleavage, without removing the jelly, were centrifuged for three minutes at a speed of 1500 revolutions per minute, and from those eggs were obtained various types of monsters besides edema. The centrifugal force is equal to 1050 times gravity. These monsters can be classified into following five types, according to their probable causes for double or triple formations. Type 1 is the triple monster (Figs.1, 2 and 3), Type 2, the incomplete double monster (Figs.4 and 5), Type 3, the double tailed monster (Figs.6 and 7), Type 4, the complete double headed monster, that is, the monster with two heads complete in structures (Fig.8), and Type 5, the incomplete double headed monster, that is, one of the heads being complete in structure while the other one being incomplete (Figs.9, 10 and 11). In none of the cases, the position of the primary invagination seems to be affected by the centrifugal force. The oil drops or something like spaces containing oil drops were not recognized in any cases. Monsters belonging to Types 4 and 5, in which the various organs in each head are formed independently, resemble the ones often repcrted hitherto. Since only one common anus is formed in these monsters, the double formation in these types may be ascribed to the splitting of the primary invagination as observed by many investigators. In monsters belonging to Tyre 3, a surplus tail, which is independent of the primary one is produced at the ventral side of the main embryo. An anus is observed to be opened at the base of this surplus tail and a gut is connected with the anus. In some cases, neural tube, myotome, pronephros and heart-like organs etc. are developed in the surplus tail. The presence of the secondary anus appears to furnish a demonstration that the surplus tail is formed by the secondary invagination at the animal half. The triple monsters (Type 1) which belong to a new type obtained by centrifuging are of interest to give a clue as to the mechanism of double formation. In specimens of this tyre, double heads are produced and a surplus tail independent of the primary one is formed at the ventral side of the main embryo. Since the double heads are considered to be produced by the splitting of the primary invagination, while the surplus tail is formed by the secondary invagination at some region apart from that of the primary invagination, the mechanism of formation of double heads and that of the double tails are different. In Type 2, some independent organs such as notochord, nerve cord and otic vecicle are formed in surplus at the ventral side of the main embryo. These organs may have been formed by a different mechanism from the splitting of the primary invagination observed in Types 4 and 5. But it is also uncertain whether they are produced by the secondary invagination observed in Type 3. Explanation of Figures. Figs.1-11. Double and triple monsters reconstructed from serial sections. Materials fixed nineteen days after centrifuging. a, anus ; c, notochord ; e, optic vecicle ; g, gut ; gb, gall-bladder ; gl, glomerulus ; h, heart ; i, infundibulum ; 1, liver ; m, myotome ; mo, mouth ; n, neural tube ; o, nose ; p, pronephros ; r, otic vecicle ; s, pancreas ; ti, tail. Throughout the figures, 1, 2 and 3 stand for the organs of the primary, the secondary and the tertiary embryos respectively.
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