TEM of a stage 2, 7-cell embryo in vivo (original magnification x7,500). Notice the beginning of internal differentiation of the nucleolus (Nl) with segregation of fibrillar and granular material (arrows).
M = mitochondrion
N = nucleus
From: Pereda and Croxatto, 1992. Reproduced with permission of the Annals, Academy of Medicine, Singapore.
Source: The Virtual Human Embryo.