Soon the cells
along the middle line of this small sheet become rearranged to make an
obvious streak or band, and about this line a simple tube is constructed
which is destined to become the future brain and spinal cord. The whole
disk continues to enlarge by further division of its constituent elements
so that it encloses more and more of the yolk mass, but the little chick
itself is made out of the cells along the central line of the original
plate, from which it folds at the sides and in front and behind so as to
lie somewhat above and apart from the flatter enclosing cell layers which
partly surround the yolk.
At the sides of the primitive nerve-tube small blocks of cells arise to
develop into primitive muscles and other structures. As nourishment is
brought to the embryo from the surrounding layers enclosing the nutrient
yolk, one system after another takes its shape and builds its several
parts into organs which can be recognized as elementary structures of a
chick. Among the more interesting ones are small clefts or slits formed in
the side walls of the rudimentary throat or pharynx. Blood-vessels go
forward from the simple heart to run up through the intervening bars
exactly as in the tadpole and the fish.
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