Polyembryony in the Armadillo: Genetic or Physiological?

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Abstract
This critical review constitutes a refutation of the Stockard-Newman theory of polyembryony in mammals. According to this, in correlation with the experimental production of double monsters in fishes, polyembryony in the armadillo is supposed to be due to a lack of oxygen during the quiescent period and delay of implantation of the embryonic vesicle. The latter is supposed to be due to a refractory phase of the corpus luteum (Stockard). The author shows that the free vesicle stage is not the one at which the embryos bud off of the germ disc. Furthermore, among hundreds of embryos in the aplacental opossum, Hartman found but a single case of polyembryony and this one in a litter of otherwise normal ova. The genetic factor is indicated by the relative constancy of 4 unisexual embryos in a litter of Dasypus novemcinctus, a variable no. (7-12) in D. hybridus. Final proof against the environmental cause of polyembryony is furnished by mammals whose embryos experience, in the vesicle stage, a delay in implantation with arrest of development, but without polyembryony[long dash]known for the deer since Bischoff (1854), for the badger since Fries (1880), and now known also for the American badger (Hamlett).

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