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 (p.v.). 

 Back of the postvelar arch is a projection in 

 which the superior commissure (s.c.) runs. An 

 early stage of the epiphysis (cp., fig. 214a) is 

 shown behind this projection, and in the projec- 

 tion is the posterior commissure (p.c). 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, i.l., fig. 213a) 

 and the vascular sacs (sacci vasculosi, v.s.). The infundibulum meets and 

 fuses with the hypophysis, an outgrowth from the buccal cavity, to form the 

 pituitary. The hypophysis may present a complex appearance, or it may be 

 comparatively simple. In Scymnus, as in Hepfanchua, it is composed of three 

 well defined divisions, an anterior terminal, a median, and a paired posterior 

 division, the posterior division being considerably removed from the body of 

 the infundibulum and connected with it only by a narrow strand. 



Figure 215 is a detailed drawing in side view of the pituitary and surround- 

 ing structures, in the adult of Squalus sucklii. In this area, in addition to the 

 inferior lobes (i.l.b.) and vascular sacs (v.s.) of the brain, there are three 

 unpaired parts of the hypophysis lying in the midventral line. These are the 



Fig. 212. Dorsal view of brain, 

 Myliohatis calif ornicus. (For ex- 

 planation see fig. 211.) 



