Observations on twin baboon embryos (Papio sp.)

Twin baboon embryos were acquired by hysterotomy from a multipara on the twenty‐sixth day of gestation. All preceding and succeeding pregnancies yielded single births. This case of dichorionic, diamniotic twins is unique because there is a seven day difference in the morphological development of the two embryos. The embryos are approximately the same fertilization age because mating was limited to one 12 hour period of the menstrual cycle. In addition only one corpus luteum was identified at laparotomy. Both twins were males as indicated by the sex chromatin, eliminating the positive identification of dizygosity. Several alternatives for the twins' origin are cited. One possibility is that there was a division of the blastomeres at the two‐cell stage, or at some other stage before implantation, yielding monozygotic twins. Another possibility is that both ova arose from the same follicle, yielding dizygotic twins. The larger embryo is 2.0 mm in greatest length and has six pairs of somites, and is similar to ten other baboon embryos 25–26 days old. The smaller embryo is of the trilaminar disc stage and is comparable in developmental features to five other baboon embryos 18–19 days old. The morphological features of each placenta are comparable in development to the age of its respective embryo.