B. Schultze (_U. anomale Duplicität der Axenorgane, Virchow's Archiv._) and Panum and Dareste (_op. cit._) hold the fusion theory-- {78} the fusion of two separate blastoderms in one ovum. Panum and Dareste have seen two separate normal blastoderms on one ovum. Allen Thompson in 1844 (_London and Edinburgh Monthly Journal of Medical Science_), Wolff, von Baer, and Reichert also observed two embryos in one ovum. Dareste is of the opinion that the fusion of two separate ova is impossible. The fission theory--the fission of a single blastoderm to make a composite monster--is supported by Wolff, J. F. Meckel, von Baer, J. Müller, Valentine, Bischoff, and others, especially by Ahlfeld. Ahlfeld says that this single blastoderm is split by pressure. Gerlach also (_Die Entstehungsweise der Doppelmissbildungen, etc.,_ Stuttgart, 1882) admits fission, but he contends that it is not so simple a process as Ahlfeld thinks it is. It is not a passive cleavage, but a result of a force in the cell-mass existing before differentiation. Gerlach calls fission at the anterior or head-end of the single blastoderm, _bifurcation_; and he has actually observed such bifurcation in a chick embryo of sixteen hours (_U. d. Entstehungsweise der vorderen Verdoppelung. Deutsche Archiv. f. klin, Med.,_ 1887). In this case the first change noticed was a broadening of the anterior end of the primitive streak; next a forked divergence appeared, and this became more pronounced; until by the twenty-sixth hour the bifurcation was half as long as the undivided posterior part. From each anterior end of the diverging branches a distinct head-process extended. Allen Thompson (_loc. cit._) in 1844 saw a goose-egg, which had been incubated for five days, in which was a double monster divided to the neck. Beyond this observation by Gerlach we have the fact, which seems to make for the fission theory, that no matter how unequally nourished or how variable in extent, the union between the halves of double monsters is always symmetric--exactly the same parts of each twin are joined. This seems to exclude a fortuitous growing together of dissimilar areas or cell-masses, for non-parasitic double terata at the least. Born ( _U. d. Furchung des Eies bei Doppelbildungen, Breslauer Aerztl. Zeitschr._, 1887), in a study of fish ova, found that ova which produce double monsters begin with a segmentation like that of the single normal ovum.