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Early Mammalian Development
In asymmetric cell division, two different types of daughter cells are formed from one parent cell. In contrast, both daughter cells formed in symmetric cell divisions are identical, but may have different fates if they are exposed to different external signals (see Figure 21-1).
Specialized sperm and egg surface proteins allow the nucleus of a single mammalian sperm to enter the cytoplasm of an egg. Fusion of a haploid sperm and haploid egg nucleus generates a diploid zygote (see Figure 21-2).
The initial divisions of the mammalian embryo yield equivalent totipotent cells, but subsequent divisions yield the first differentiation event, the separation of the trophectoderm from the inner cell mass (see Figures 21-3 and 21-4).