To investigate the effect of the decrease in the number of embryonic cells on the development and morphogenesis of mouse embryos at the peri-implantation stage, half embryos which developed from bisected morulae were morphologically classified into eublastocysts (eu-blasts), pseudo-blastocysts (pseudo-blasts), trophectodermal vesicles (TVs) and non-integrated forms (NIFs) and then cultured on plastic substratum, uterine-epithelial-cell monolayer and type I collagen gel. When half embryos were cultured on plastic substratum and cell monolayer, the rates of attachment and trophoblast outgrowth in the eu- and pseudo-blasts were not significantly different from those of the controls. The TVs and NIFs showed significantly lower rates of outgrowth than the controls (P<0.01). When half embryos were cultured on type I collagen gel, no significant difference was observed in the rate of primitive endoderm formation between the eu-blasts and controls after 36 hr of culture. In the eu-blasts, however, the developmental rate to the egg cylinder at stage 8 was significantly lower than in the controls after 72 hr (P<0.05). The pseudo-blasts revealed significantly lower rates of endoderm formation and development to the egg cylinder than the controls after 36 and 72 hr, respectively (P<0.05). In the TVs and NIFs, the rates of outgrowth were significantly lower than those of the controls (P<0.05) and no egg cylinder was observed. The invasion of type I collagen gel by the cytoplasmic protrusions of trophoblast cells was observed regardless of the type and developmental stage of the embryos. The results show that a decrease in the number of embryonic cells affects the formation of the primitive endoderm and the development to the egg cylinder in vitro.