LM of a stage 1c embryo (zygote) in vitro, 20 hours post-insemination. Pronuclear envelopes have disappeared. Remnant of one pronucleus is faint. Syngamy is beginning.
From: Veeck and Zaninovic´, 2003. Reproduced with permission of the publisher, Taylor & Francis Group.
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Source: The Virtual Human Embryo.