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 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.