Postimplantation development of mouse blastocysts with two separate inner cell masses.

Blastocysts with double inner cell masses (ICMs) were produced by electrofusion of two blastocysts and transplanted to pseudopregnant recipients. The implanted embryos were either examined histologically (8th day of pregnancy) or dissected and inspected in toto (11th or 12th day). In most cases both ICMs of experimental blastocysts developed into separate egg cylinders. Both cylinders were located in the common yolk sac cavity. Some cylinders were quite normal, but most of them were small and deprived of embryonic membranes. Ectoplacental cones of these cylinders were often oriented laterally or even antimesometrially. The development of cylinders seems to depend upon the position of their ectoplacental cones--cylinders with cones situated antimesometrially were handicapped in development. Among four sets of twin embryos examined on the 11th or 12th day, in one set each of the twins were equally developed, and in the three others one of the twins was more advanced.