When blastocysts from pregnant mice 3½ days p.c. were transferred to the uteri of synchronous pseudopregnant recipients, the time of onset of implantation (judged by the appearance of pontamine blue reactivity in the uterus) was not altered by removal of the zona pellucida. The competence of zona-free blastocysts to elicit the pontamine blue reaction was similar to that of control blastocysts. The low implantation rate of eight-cell embryos from donors 2½ days p.c., transferred to recipients 3½ days p.c., was not increased by previous removal of the zona. It was concluded that, in mice, the decidualizing stimulus from the blastocyst to the uterus stems neither from the zona itself, nor from cellular contact between the vitelline surface and the uterine epithelium.
Removal of the zona by treatment with pronase reduced the viability of transferred blastocysts, so that by mid-gestation the proportion of zona-free blastocysts represented by live embryos was significantly below the control level.