f 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. In brief, the young chick
possesses a series of gill-slits, for these structures are the same in
essential plan and relations as the clefts of tadpoles and fishes. Does
this mean that even birds have descended from gill-breathing ancestors?
Science answers in the affirmative, because evolution gives the only
reasonable explanation of such facts as these. The case seems different
from that of the frog, because gills are used by the tadpole, but
gill-slits and gill-bars can have no conceivable value for the chick as
organs concerned with the purification of the blood. None the less, if the
transition from a gilled tadpole to the adult with lungs means an
evolution of amphibia from fishlike ancestors, then the change of a chick
embryo with gill-clefts into the fledgling without them is most reasonably
interpreted as proof that birds as well as amphibia have had ancestors as
simple as fishes.

As development progresses four small pads make their appearance; two of
these lie on either side of the body back of the head and the other two
arise near the posterior end. They are far from being wings and legs, but
as day follows day they become molded into somewhat similar limbs, as much
alike in general plan as the four legs of a lizard; subsequently the ones
at the front change into real wings and the hinder ones become legs.
Meanwhile the internal organs slowly transform from fishlike structures
into things that display the characteristics of reptilian counterparts,
and only later do they become truly avian. Last of all the finishing
touches are mad