The rate of development of cultured zygotes compared with embryos collected from the reproductive tract at the same time after the administration of ovulatory hCG (h). No embryos could be flushed from the uterus at 113 h since implantation had commenced (not applicable). Numbers shown in brackets are the number of embryos examined. (A) The proportion of embryos that were morphological blastocysts. (B) The total number of cells present in the blastocysts shown in Fig.1A (mean + SEM).