The tough but glassy-looking membrane that starts to surround the egg (the primary oocyte) while it's still in the follicle, protects it against sperm which have not undergone the acrosome reaction at its surface, and keeps the cells of the early embryo together until the embryo, as a blastocyst, hatches through it in preparation for implantation.

fertilised egg or ovum up to the first cleavage division; this stage of development of the embryo takes about 22 hours, and for the last few of these hours it's at the pronuclear stage.

(ZIFT) A form of assisted conception in which in vitro fertilisation (IVF) is used to produce fertilisation; transfer is made on the day after egg retrieval and IVF, before the fertilised egg divides (it's at the zygote, or pronuclear stage). Useful if the potential fertilising ability of sperm is in doubt. The transfer is made to the fallopian tube to obtain advantages similar to gamete intrafallopian transfer (GIFT) and is made possible by laparoscopy (with anesthesia) or by transvaginal ultrasound (without anesthesia). Synonymous with pronuclear-stage transfer (PROST).