What seems to make for the fission theory is that in non-parasitic double terata, no matter how unequally nourished or how variable in extent, the union between the halves of double monsters is symmetric, and the same part of each twin is joined. This fact is used as a reason to exclude a fortuitous growing together of dissimilar areas of cell-masses, at least in non-parasitic cases. Born,[58] in a study of fish ova, found that eggs which produce double monsters begin with a segmentation like that of the simple normal ovum. Composite spermatozoa have been observed with two and three heads and one body and tail-piece, but the significance of these abnormal cells is not known. [58a] _U. d. Furchung des Eies bei Doppelbildungen. Breslauer Aertzliche Zeitschrift_, 1887. Embryos of sea-urchins in the two-cell and four-cell stages can be separated by shaking into isolated blastomeres, and the segments will grow into full though dwarfed larvae. The same division with the growth of dwarfed larvae has been made in Amphioxus, in the teleost Fundulus, in Triton, in a number of Hydromedusae and several other low forms of life. When the division is not made completely double monsters result. Up to a certain stage of development the blastomeres of the Medusa embryo are totipotent, or capable of developing into any part of the body. The limitation of development in a particular case lies in the cytoplasm rather than in the nuclei of the cells. If frogs' eggs are fastened in abnormal positions, inverted or on the side, a rearrangement of the egg material results, wherein the nucleus and cytoplasm rise and the deutoplasm sinks. This change of axis shifts the embryo. If an egg is turned upside down in the two-cell stage, a whole embryo, or half a double embryo, may arise from each of the two blastomeres, instead of a normal half-embryo. A half-embryo or a whole dwarf may arise according to the artificial position of the blastomere. Each of the two blastomeres contains all the materials potentially for the formation of the whole body, and these materials build up a whole body or a half body according to the grouping they take on. Primarily the egg cytoplasm, in low forms of animal life, is totipotent; it has no fixed relation with the parts to which it gives rise, and may be artificially modified or differentiated. These effects, from position and traumatic dislocation, suggest explanations for teratic forms in higher animals.