The elasmobranch fishes (1934) The elasmobranch fishes . elasmobranchfish03dani Year: 1934 THE ELASMOBRANCII FISHES 151 opened jioeket in HcptdiicIiHs. In tlie rays the pocket is somewhat like an in- verted U, in which the internal and external branchial apertures are repre- sented by the ti])s of the IT close together. The first indication of a pocket in the embryo appears as an evagination or outpocketing of the pharyngeal wall toward the exterior (gp., fig. 144). As this approaches the surface it meets a slight pitting in from the outside, and the wall between the two breaks through to for
The elasmobranch fishes (1934) The elasmobranch fishes . elasmobranchfish03dani Year: 1934 THE ELASMOBRANCII FISHES 151 opened jioeket in HcptdiicIiHs. In tlie rays the pocket is somewhat like an in- verted U, in which the internal and external branchial apertures are repre- sented by the ti])s of the IT close together. The first indication of a pocket in the embryo appears as an evagination or outpocketing of the pharyngeal wall toward the exterior (gp., fig. 144). As this approaches the surface it meets a slight pitting in from the outside, and the wall between the two breaks through to form the external branchial cleft or aperture. In pentanchid types six of these pouches, including the s])iracular, are formed; but accessory ]>ockets are often indicated. Thus in Acanthias a small i^oucli on the left side (or a pair of pouches) is produced as an evagina- tion from the floor of the pharynx just back and mediad of the sixth pouch. This pouch does not reach the outside layer but comes in contact with the roof of the pericardial cavity. Here it forms numerous tubules and becomes the so-called post- branchial or suprapericardial body. These bodies have also been described for Scyllium, Galeus, Pristiurus, and Raja. In figure 144 the anterior clefts have thus broken through. Between the pockets are columns, from which all the tissues of the holobranchs are later produced including their supporting cartilages and musculature. On the posterior wall of the hyoidean cleft will appear a little later the begin- nings of the embryonic or external gill filaments. The whole column from the internal branchial arch toward the exterior lengthens out and the central core becomes a plate, the septum or dia- phragm of the gill. This plate supports the fila- ments or respiratory membrane, and is peculiar in the Elasmobranchs in that it extends outward beyond the filaments which are attached to it. It is from the peculiar attachment of the filaments to the septa that the Elasmo- bra
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