A number of workers, including Dareste (1877), Mitrophanov ( '00)' Tannreuther ( '19), Newman ( '40)' and others, have reported and described various types of monsters and duplicities in the chick embryo. Nalbandov ( '42) reported one case of viable twin chicks, presumably of monozygotic origin. Stockard ('21), stated, "that in spite of many experimental studies on the developing hen's eggs by Dareste, Fere, the writer, and others, no double monster, or twin conditions have been produced." Twieselmann ( '35) and others have produced doubling in the chick embryo by mechanical injury to the blastoderm, and Waddington and Schmidt ('33) have induced secondary embryos by transplantation of organizer material. Stockard's theory that an arrest of development in pregastrula-stage embryos caused twinning and double monsters was tested by Riddle ('23) on the pigeon and ring dove. He obtained eggs from these species at various stages of development before gastrulation (4 to 24 hours premature) and, after precooling them, subsequently incubated them. His attempts to produce twins or double monsters were unsuccessful.
Sturkie and Williams ( '45)' who arrested development in the hen's egg prior to gastrulation (134 to 154 hours after fertilization) reported two cases of embryonic duplications,