The literature of Duplicity in Birds affords, out of a total of about ninety-five recorded cases of multiple formations of all kinds on a single blastoderm, from the stage of the primitive streak to the fourth day of incubation, only a small proportion of instances of “duplicitas anterior.” Dareste (i.) in his atlas figures three; Gerlach (ii.) adds representations of three others—one case of his own, a second originally described by Ahlfeld, and a third by Reichert; Klaussner (iii.) gives a seventh case; and Bianchi (iv.) describes a monstrous embryo at a later stage (1°5 cm. in length).

Most observers have been content with the partial information derived from the study of the whole object, and only three embryos of this class, which have been studied in serial sections, have been described:—

1st. Erich Hoffman's (v.) with three somites.

2nd. Mitrophanow's (vi.) with six somites.

3rd. Kaestner's (vii.) with seven somites.