Production of identical twins by bisection of blastocysts in the cow

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Abstract
Day-8 embryos were recovered by a non-surgical method from superovulated crossbred heifers. Normal expanded blastocysts with a distinct inner cell mass and a trophoblast were released from the zona pellucida and bisected along a sagittal plane into 2 half blastocysts. Each half blastocyst was replaced in an empty zona pellucida and cultured for 2 h in B2 medium. After culture the half blastocysts were directly transferred to recipient heifers via the cervix. From 11 blastocysts, 11 monozygotic half blastocyst pairs were transferred to 11 recipients: 8 recipients became pregnant, 4 carried twins and 1 delivered a normal calf and an acardiacus amorphus monster consisting of disorganized embryonic tissues. A further 11 half blastocysts were transferred as singletons to 11 recipients. Five recipients were apparently pregnant at day 42. One returned to estrus at day 45, 3 were carrying normal fetuses and 1 a pair of normal twin fetuses when slaughtered at day 128. Even after the 1st irreversible cellular differentiation which occurs at the blastocyst stage, it is still possible to produce identical cattle twins by bisection of the day-8 blastocyst.