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In this chapter we focus mainly
on the fertilization of the egg to form a zygote,
and the subsequent development of the young animal
up to organogenesis.
Differentiated cells must become
ever more organized, determining the elaborate
pattern of tissues and organs of a multicellular
animal. This development of form, known as morphogenesis,
progresses through pattern formation, steps
requiring signaling among cells, modifications
in the shapes of some cells, cell migrations,
communications with the extracellular matrix,
and even apoptosis (programmed cell death)
of some cells.
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