established by comparative anatomy as the evolutionary sequence.

However it is well to review some of the changes by which a chick comes
into existence before attempting to comprehend fully the fundamental
principle of development that the tadpole's history discloses to us. The
egg of a common fowl is certainly not a chick. Within the calcareous shell
are two delicate membranes that enclose the white or albumen; within this,
swung by two thickened cords of the albumen, is the yellow yolk ball
enclosed by a proper membrane of its own. In the earliest condition, even
before the albumen and the shell are added and before the egg is laid, on
one side of the yolk-mass there is a tiny protoplasmic spot which is at
first a single cell and nothing more. The hen's egg is relatively
enormous, but nevertheless, like that of the frog, it starts upon its
course of development as a single unitary biological element--a cell.
During the earliest subsequent hours the first cell divides again and
again to form a small disk upon the surface of the yolk. 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