214 



COMPARATIVE ANATOMY 



becomes constricted off from the brain and disappears during 

 development. 1 Lobi inferiores, as well as a hypophysis and a 

 glandular saccus vasculosus are present, but these vary much in 

 the degree of their development. The saccus vasculosus here too, 

 opens by several apertures into the infundibulum and is surrounded 

 by a blood-sinus. 



The mid-brain and cerebellum are extremely large relatively ; 

 the latter is bent upon itself, overlies the medulla oblongata behind, 



and is prolonged in front into the ventricle 

 of the mid-brain as a valvula cerebelli 

 (Fig. 161), as in Ganoids. 



The Teleostean brain is a further 

 specialisation of the type seen in Ganoids, 

 and has no direct connection with that of 

 Cyclostomes or Elasmobranchs. 



Dipnoans. Both as regards external 

 and internal structure, certain points of 

 resemblance may be seen between the 

 brain of Dipnoans and that of Elasmo- 

 branchs and Ganoids, but in other respects 

 it is specialised. In Ceratodus, there is a 

 considerable space between the walls of 

 the cranium and the brain except in the 

 region of the large olfactory lobes. The 

 telencephalon is well developed, and the 

 thin pallium, which is mainly nervous, is 

 involuted along the median longitudinal 

 line so as to completely separate the two 

 hemispheres from one another dorsally in 

 Protopterus and Lepidosiren: in Ceratodus 

 they are partly united together dorsally and 

 posteriorly by a narrow bridge formed by 

 the choroid plexus. Olfactory lobes arise 

 from the telencephalon anteriorly, and contain ventricles : in 

 Ceratodus they overlie the hemispheres. Postero-laterally each 

 hemisphere gives rise to a distinct hippocampal lobe. The pineal 

 body has a long stalk, and its distal vesicle perforates the carti- 

 laginous roof of the skull : in the embryo Ceratodus it even 

 reaches as far as the integument. The complicated choroid plexus 

 in this region gives rise to a large vesicle over which the pineal 

 stalk extends. Lobi inferiores are present. 



The well-marked mid-brain is indistinctly paired in Ceratodus, 

 but is unpaired in Protopterus and Lepidosiren. The cerebellum 



1 A parietal foramen is present in the embryo in several Teleosts (e.y. 

 Cottus, Salmo), and in some others (e.y. Callichthys) persists in the adult without 

 a corresponding development of the pineal organ. In certain deep-sea forms (e.g. 

 Argyropelecus, Cyclothone), however, the latter is comparatively complicated, 

 and, as in Petromyzon, consists of two vesicles, showing regressive characters. 



FIG. 162. - TRANSVERSE 

 SECTION THROUGH THE 

 FORE-PAKT OF THE TELE- 

 OSTEAN BRAIN. 



G.t, corpora striata ; Ep, 

 ependyme ; fr, frontal 

 bone, underneath which 

 the pineal tube, Gp, is 

 visible in transverse sec- 

 tion, and below this the 

 perimeningeal tissue, Pm; 

 Pa, the pallium, formed 

 of a simple epithelial 

 layer ; T, T, olfactory 

 tracts ; V.cm, teloccele. 



