of nutrition, there must result an increased power of development and growth.
By the absorption of fluid from the maternal tissues in which it is imbedded and the accumulation of this fluid at the center of the mass, the cells of this mulberry-like body become crowded outward to the periphery, thus forming a lining for the membranous sac—i. e., the outer covering of the ovum—which incloses them, the entire globular mass now being about one twenty-fourth of an inch in diameter, and consisting of a structureless outer membrane lined with a layer of nucleated cells (the blastoderm), and filled with clear fluid. These lining cells multiply rapidly; the inner ones become larger, darker, and softer than the outer ones, and thus differentiation has again occurred—the lining having developed into two distinct layers. This is known as the gastrula stage of embryonic development. All animals, from sponges to man, pass through this phase, becoming first two and then three layered sacs; but, from this point, the different branches or sub-kingdoms diverge; and the next recognizable phase in the development of the human embryo is confined to vertebrates, with a single exception, the ascidian. The larval ascidian swims like a tadpole by means of a caudal appendage in which may be traced a rod-like body thought to be a rudimentary chorda dorsalis, since it resembles the embryonic structure which, in the perfect vertebrate, develops into the spinal column with its contained, highly endowed spinal cord. This, however, not only fails to develop but actually disappears in adult life, leaving the ascidian a simple invertebrate animal. But, whether the ascidian be a true connecting link between invertebrates and vertebrates, or, as suggested by Balfour, a reversion from the higher form, it serves equally to indicate a close relationship between these two great subdivisions of the animal kingdom.
Between the two layers of germinal cells which belong to the gastrula stage, a third layer is developed, and from these three layers (the epiblast, the mesoblast, and the hypoblast) all the tissues and organs of the body are derived. The inner layer (hypoblast) gives origin to the epithelial lining of the alimentary canal and to the various glands derived from it. From the outer layer (epiblast) are developed the brain and spinal cord, and the epidermis with its appendages and derivatives, including the organs of the special senses. From the middle layer (mesoblast) the various intermediate structures are produced. The remaining history of development is, therefore, the history of the differentiation of these three layers of the blastoderm (which alike consist of simple nucleated cells) into the various tissues and organs of the body. Accompanying this process there is a corresponding development of functions. As absorption and assimilation, so perfectly performed by these germinal cells, are, however, the fundamental facts in the nutrition of even the highest organisms, so also reaction in response to a stimulus, of which we have found even the moner and the
