Light micrograph of a stage 3 embryo in vivo obtained by uterine lavage after a single, periovular insemination. The embryo was then transferred to the recipient uterus of an infertile woman where it resulted in an intrauterine pregnancy and a normal, term infant. A blastocystic cavity and inner cell mass are clearly visible.

(From: Buster et al., 1985. Reproduced with permission from the publisher, Elsevier.)