All these changes are familiar and natural, but until science places them and similar facts in their proper relations their significance is lost to us. The tadpole is essentially a fish in its general structure and mode of life, even though its heritage is such that it can develop into a higher animal. 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 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