A chimera can be defined as an individual composed of cells derived from two or more separate zygotes. The incorporation of the two cell lines into the embryo can occur spontaneously or be produced experimentally. It can take place at various periods of embryogenesis and in various ways. It seems that in mammals three mechanisms can be involved (see Tarkowski 1969 for a detailed discussion and references). Double fertilization combined with immediate cleavage of the oocyte at the first or second meiotic division Both dispermy and immediate cleavage have been described but simultaneous occurrence of both events in one egg has not, as yet, been observed, nor experimentally reproduced.