At or before 36 h (HH stage 10), the three germ layers are present, the body axis is established and anteroposterior patterning is well under way, left-right asymmetry is established early during gastrulation, and dorsoventral patterning is ongoing. Somites are being continuously generated and will give rise to the dermis, musculature, and axial skeleton. Neural crest migration has commenced to give rise to derivatives that include the nonplacodal parts of the peripheral nervous system, cranial skeletal elements, some smooth muscle, and melanocytes.
During the third day of development (HH stages 13-19), the head begins to rotate to the left, and the optic vesicle, otic vesicle, nasal pits, branchial arches, and pituitary begin to develop. The limb buds begin to form and project from the trunk during this period, and the amnion extends over the embryo and closes. During the following 48 h or so, the majority of the organ systems have been established such that between the sixth day and hatching, much of development is concerned largely with increase in size of existing organs, although in the case of the nervous system, this is accompanied by considerable increase in complexity.
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