first - cleavage mouse embryos , fertilized in vitro and in vivo , provide ideal material for chromosomal analysis . with the appropriate incubation in a mitotic inhibitor 
 , syngamy is prevented and the sperm- and egg - derived chromosomes remain as separate clusters . because the latter chromosomes undergo condensation sooner than those from the spermatozoon , the parental source of chromosome sets can be identified even without a marker chromosome . thus these embryos can be analyzed both for the primary incidence and the parental source of a number of chromosomal anomalies , including aneuploidy . by using fertilization in vitro to obtain the embryos , 
 the synchrony of fertilization and nuclear development is such that 80% or more of the chromosomal preparations are suitable for analysis , compared with about 50% for embryos fertilized in vivo.imagesfigure 1.figure 2.figure 3.figure 4 .