Editing Developmental Biology (section)

== 1. Embryogenesis: Establishing the Body Plan ==

Embryogenesis is the early stage of development following fertilization. While the specific details vary greatly across the animal kingdom, the fundamental phases remain remarkably consistent.

* '''Cleavage:''' Immediately after fertilization, the zygote undergoes a series of rapid mitotic cell divisions without any overall growth in the size of the embryo. This partitions the large zygote into many smaller cells called blastomeres, forming a solid ball called a morula.
* '''Blastula Formation:''' As cleavage continues, the cells secrete fluid into the center of the morula, creating a hollow cavity (the blastocoel). At this stage, the embryo is referred to as a blastula (or a blastocyst in mammals).
* '''Gastrulation:''' This is arguably the most critical event in early development. The single-layered blastula undergoes massive cellular migrations and folding, reorganizing itself into a multi-layered structure called the gastrula. This establishes the three fundamental embryonic germ layers:
** ''Ectoderm (Outer Layer):'' Gives rise to the nervous system and epidermis (skin).
** ''Mesoderm (Middle Layer):'' Forms the skeletal system, muscles, circulatory system, and kidneys.
** ''Endoderm (Inner Layer):'' Becomes the lining of the digestive and respiratory tracts, as well as organs like the liver and pancreas.