THE BRAIX 



165 



connection with that of Cyclostomes or Elasmobranchs, but has 

 certainly passed through Ganoid-like stages. 



N 



rh 



pros 



FIG. 135. TRANSVERSE SECTION THROUGH THE FORE PART OF THE TELEOSTEAN 



BRAIN. 



fr, frontal bone, underneath which the pineal tube, Gp, is visible in transverse sec- 

 tion, and below this the pia mater, Pm ; Pa, the pallium, or roof of the sec- 

 ondary fore-brain, formed of a simple epithelial layer ; V.cm, prosocoele ; Ep, 

 ependyme ; T, T, olfactory tracts at the base of the corpora striata (C.st.). 



Dipnoi. Both as regards external and internal structure, 

 certain points of resemblance may be 

 seen between the brain of Dipnoans 

 and that of Elasmobranchs on the 

 one hand and Amphibians on the other. 

 This fact probably indicates that 

 though the Elasmobranchii and Dipnoi 

 have arisen from a common ancestral 

 type, they have become differentiated 

 along different lines. 



The prosencephalon is well de- 

 veloped (Fig. 136) : the thin pallium 

 is mainly nervous, and is involuted 

 along the median longitudinal line so 

 as to completely separate the two 

 hemispheres from one another in 

 Protopterus : in Ceratodus they are 

 united together posteriorly by a narrow 

 commissure. Olfactory lobes arise 

 from the prosencephalon anteriorly, 

 and contain ventricles. 



The thalamencephalon of Pro- 

 topterus presents certain very charac- 

 teristic features, especially as regards 

 its roof. The pineal body has a long 

 stalk, and its distal vesicle perforates 

 the cartilaginous roof of the skull : 

 in the embryo Ceratodus it even 

 reaches as far as the integument. 

 The choroid plexus gives rise to a 

 vesicular organ, and as regards its 



FIG. 136. BRAIN OF Ceratodus 

 fosteri. Dorsal view. (From 

 Parker and Has well's Zoology. ) 



aud, auditory nerve ; cbl, cere- 

 bellum ; fac, facial nerve ; gl, 

 glossopharyngeal ; med, me- 

 dulla oblongata ; mes, mesence- 

 phalon ; oc, oculomotor nerve; 

 opt, optic nerve ; pros, cere- 

 bral hemispheres ; rh, olfac- 

 tory lobes ; vg, vagus nerve. 



