The elasmobranch fishes . elasmobranchfish03dani Year: 1934 THE ELASMOBRANCH FISHES 233 greater compaetness of brain obtains, or in those where the eerebellnm is especially well developed, the diencephalon may be more or less completely hidden (Scoliodon, fig. 211a; Mijliohatis, fig. 212). Both dorsally and ventrally the diencephalon is characterized by out- growths which are of interest. From the roof arises the chimney-like epiphysis (pineal stalk) {ep., fig. 213a) which passes upward and forward to the roof of the cranium. In general the stalk terminates at the roof immediately posterior t
The elasmobranch fishes . elasmobranchfish03dani Year: 1934 THE ELASMOBRANCH FISHES 233 greater compaetness of brain obtains, or in those where the eerebellnm is especially well developed, the diencephalon may be more or less completely hidden (Scoliodon, fig. 211a; Mijliohatis, fig. 212). Both dorsally and ventrally the diencephalon is characterized by out- growths which are of interest. From the roof arises the chimney-like epiphysis (pineal stalk) {ep., fig. 213a) which passes upward and forward to the roof of the cranium. In general the stalk terminates at the roof immediately posterior to the anterior fontanelle and is usually spread out terminally into the disc-like pineal body. In development the pineal region in the early embryo of Acanthias (Minot, 1901) shows a series of arches in the roof of the brain which are separated by a series of projections. The long anterior projection (v., fig. 214) is the velum, an important landmark separating telencephalon from diencephalon. The velum in figure 214b separates the paraphysial arch (pa.) from the small postvelar arch (). Back of the postvelar arch is a projection in which the superior commissure () runs. An early stage of the epiphysis (cp., fig. 214a) is shown behind this projection, and in the projec- tion is the posterior commissure (). Figure 214b is a later stage in the development of the pineal stalk in which the surrounding struc- tures have reached a definitive form. The pineal stalk (ep.) has almost reached the surface; the paraphysis (pa.) is enlarged; and the velum and commissures are well marked. From the floor of the diencephalon an evagi- nation, the infundibulum (in., fig. 213b), drops downward and backward, and at its sides are the inferior lobes (lobi inferiores, , fig. 213a) and the vascular sacs (sacci vasculosi, ). The infundibulum meets and fuses with the hypophysis, an outgrowth from the buccal cavity, to form the pituitary. The hypophysis may present a complex appearanc
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