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Show 18 DR. J. F. GEMMILL ox [May 12, in this case the two bladders lie in the same plane and have corresponding right and left sides. But the right side of BL' is in connection with a left Wolffian duct {Wda), while the left side is in connection with a right Wolffian duct. Such a transposition is exceedingly rare in double monstrosities. Figure 26 was taken from a case of symmetrical ventral union. Two vents and two urinary pores were present, and as they opened laterally in pairs towards opposite sides, they lay in a plane at right angles to the sagittal plane of the twin bodies. This arrangement has many parallels in teratology, e. g. in cases of ischiopagous double monstrosity*. It preserves the natural correspondence between rights and lefts in the ducts and bladders, which, as has just been seen, is inverted in the case from which figure 25 of Plate III. was taken. GENERAL. With the rarest exceptions, all double monstrosities in fishes are examples either of anterior duplicity or of union by the yolk-sac. The explanation of this remarkable fact seems to m e to be as follows :-In all these cases, two centres of gastrulation form on the edge of the blastoderm at a greater or less distance from one another. The spreading of the blastoderm over the yolk-mass goes on freely all round except at and near the primitive streak. There, changes take place which lead to increase in length of the embryonic axis, and which are interpreted by many as concrescence. If the two centres of gastrulation happen to be near one another, the whole of the blastoderm edge separating them eventually will be used up in the process of concrescence; the later formed parts of the embryonic axes will be drawn closer and closer to one another, until in their turn the axes themselves coalesce. The degree of union will be in inverse proportion to the original distance from one another of the two centres of gastrulation. Should the two centres of gastrulation be so far apart that the middle portion of the intervening blastoderm edge is not involved in concrescence but is left free to extend over the yolk-mass, the two embryonic axes will be independent along their whole length, and the only structures which connect them will be the blastoderm and the yolk-sac. According to this view, double monsters showing anterior duplicity are the result of what may be called primary fusion, that is, concrescence of their growing embryonic axes. In birds, typical concrescence can occur only during the earliest stage of formation of the primitive streak, i. e. so long as the groove of the sickle and knob is open. Any subsequent concrescence can take place only by the incidental drawing in and utilisation of lateral blastema at the growing zone. This process * J. P. Gemmill in ' Journal of Anatomy and Physiology,' vol. xxxvi. p. 263. |