Fig. 13. High power TEM of stage 4 embryo in vitro. Measuring bar = 0.5 µm. CTr, cytotrophoblast cell; Ep, epiblast cell; N, nucleus of epiblast cell. Notice the long slender projection of the cytotrophoblast cell (arrows) that invaginates the plasmalemma of the epiblast cell near its nucleus. (From: Lindenberg et al., 1986. Reproduced with permission of the publisher, Oxford University Press.)