When it does become a frog it proves beyond a doubt that there is
no impassable barrier between fishes and amphibia. Our earlier comparison
of the structures of these two classes of vertebrates led to the
conclusion that the latter had evolved from antecedents like the former,
and had thus followed them upon the earth; now that sequence seems to have
some connection with the method by which a tadpole, obviously not a fish
but nevertheless actually fishlike, changes into a frog, a member of a
higher class of vertebrates. This method is employed by developing frogs
apparently because it follows the ancestral order of events, and because,
so to speak, the only way a frog knows how to become a frog is to develop
from an egg first into a fishlike tadpole and then to alter itself as its
ancestors did during their evolution in the past. We begin to see, then,
that in addition to the impressive fact of development itself, the mode of
organic transformation is far more conclusive evidence of evolution,
because it reveals an order of events which parallels the order
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 o