It has been shown during the last decade that double embryos may be produced artificially by incomplete separation of the first two blastomeres. Driesch, Loeb, and others have demonstrated that if the first two cells of the egg of the sea-urchin be incompletely separated, each may produce a single embryo and the two remain sticking together. Wilson has shown in amphioxus that the same result occurs if the first two cells are partially separated by shaking. Schultze has shown in the frog that if at the two-cell stage the egg is held in an inverted position, _i.e._ with the white hemisphere turned upwards, each blastomere gives rise to a whole embryo--the two embryos being united, sometimes in one way, sometimes in another, as shown in Fig. 63. In this case it appears that the results are due to a rotation of the contents of each blastomere, so that like parts of the two blastomeres become separated. In the egg of the sea-urchin, and of amphioxus, gravity does not have a similar action on the egg, but the results seem to be due to a mechanical separation of the blastomeres. These cases of double structures, produced by the segmenting egg, appear to belong to the same category as those described above for adult forms--especially in those cases where pieces regenerate by morphallaxis. [Illustration: FIG. 44.--_A._ _Planaria lugubris_, cut in two as far forward as region between eyes, regenerating half-heads. _B._ Same cut in two at one side of middle line. Smaller piece produced a new head. _C._ _Planaria maculata_, split in two. It produced two heads in angle. _D._ Another, that produced a single head in angle.]