Armadillos and the issue of specific polyembryony
Susana V. García
The armadillos are small mammals of South and Central America, known for having a bony armor shell. The nine-banded armadillo (Dasypus novemcinctus) is the most widespread of this family that expanded to Texas in the 1850s. This species and the "mulita" (Dasypus hybridus) from Argentine and Paraguay are today the only known mammals that exhibit "specific polyembryony"; i.e. a process in which the fertilized ovum regularly gives rise to more than a single embryo.
In the late nineteenth century, some naturalists in Europe observed one or two pregnant uteruses of preserved nine-banded armadillo and found a curious phenomenon: the four foetuses were surrounded of a common chorion, a fact that contrasted to the known fact that every single mammalian foetus possessed its own chorion.
In South America there was a popular belief that in a certain species of armadillo all of the offspring from one mother was either male or female, a curious phenomenon mentioned by European travellers and naturalists. In the 1880s, the German zoologist Hermann von Ihering tested this belief in Brazil, by observing two pregnant females of mulita obtained in Paraguay. Each female contained eight foetuses and they were in fact all males. Von Ihering surmised that they originated from a single egg. In 1901, a different hypothesis was proposed by the Polish researcher Aleksander Rosner, on the basis of a histological examination of the ovaries of one nine-banded armadillo sent by Ihering from Brazil. Rosner found polyovular follicles in this specimen; he proposed that the four foetuses were formed of four ova discharged together and that the common chorion derived from the union of the contiguous walls of the embryos. Later observations showed that Rosner's material was pathological or quite exceptional.
Reference: García, Susana . 2007. Armadillos and the issue of specific polyembryony. The Virtual Laboratory (ISSN 1866-4784), http://vlp.mpiwg-berlin.mpg.de/references?id=art72