The alimentary canal of all vertebrates forms a tube stretching the whole length of the animal; the anterior part of this tube becomes pouched on each side at regular intervals, and the walls of each pouch becoming folded form the respiratory surfaces or gills. The openings of these separate pouches into the central lumen of the gut form the internal gill-pouch openings; the other extremity of the pouch approaches the external surface of the animal, and finally breaks through to form a series of external gill-pouch openings. From the mesoblastic tissue, between each gill-pouch, there is formed a supporting cartilaginous bar, to which are attached a system of branchial muscles, with their nerves and blood-vessels. These cartilaginous bars, in all fishes above the Cyclostomata, form a supporting framework for the internal gill-slit, so that the gills are situated externally to them; the more primitive arrangement is, as already mentioned, a system of cartilaginous bars, extra-branchial in position, so that the gills are situated internally to them. From this description of the mode of formation of the respiratory apparatus in water-breathing vertebrates the conception has arisen of the gill-pouch as the branchial unit, a conception which is absolutely removed from all idea of a branchial unit such as is found in an arthropod, viz. an appendage. This conception of spaces as units pervades the whole of embryology, and is the outcome of the gastrula theory--a theory which {166}teaches that all animals above the Protozoa are derived from a form which by invagination of its external surface formed an internal cavity or primitive gut. From pouches of this gut other cavities were said to be formed, called coelomic cavities, and thus arose the group of coelomatous animals. To speak of the developmental history of animals in terms of spaces; to speak of the atrophy of a cavity as though such a thing were possible, is, to my mind, the wrong way of looking at the facts of anatomy. It resembles the description of a net as a number of holes tied together with string, which is not usually considered the best method of description. There are two ways in which a series of pouches can be formed from a simple tube without folding, either by a thinning at regular intervals of the original tissue surrounding the tube, or by the ingrowth into the tube of the surrounding tissue at regular intervals, thus-- [Illustration: FIG. 67.--DIAGRAMS TO SHOW THE TWO METHODS OF POUCH-FORMATION. A, by the thinning of the mesoblast at intervals. B, by the ingrowth of mesoblast at intervals. _Ep._, epiblast; _Mes._, mesoblast; _Hy._, hypoblast.]