THE BRAIN. 137 



the part described on page 132 as the peripheral region 

 (pallium) of the prosencephalon consists of a single layer of 

 epithelial cells. This is covered on its dorsal surface by the pin 

 inater, and thus here, as is also the case with the secondary 

 fore-brain of Teleostei (see Fig. 113), there is a persistence of 

 that low stage of development in which the prosencephalon is re- 

 presented by a thin- walled and dorsally unpaired vesicle; that is, 

 there is no separation into two hemispheres by a cleft in the 

 peripheral region. In Fig. 110 the peripheral region is not in- 

 dicated, and thus the central portion of the prosencepha'.on is seen, 

 the floor of the latter, or corpora striata, being exposed. The 

 olfactory lobes (LoT) are connected with the corpora striata anteriorly. 



Lot 



Ifed. 



FIG. 110. BKAIX UK Ammocoetes. (Dorsal view.) 



Lol, olfactory lobes and nerves (/) ; VK, basal portion of the prosencephalon ; Zll, 

 thalamencephalon, with the pineal gland (Z) ; MH, mesencephalon ; HIT, meteu- 

 cephalon, on each side of which is seen the origin of the trigeminal ( V) XH, 

 myelencephalon ; Fr, fourth ventricle ; VII, VIII, the points of origin of the 

 facial and auditory nerves ; Mcd, spinal cord. 



The meten- and myelencephalon of Ammocoetes are remark- 

 ably long, while in Petromyzon and Myxine, the individual 

 portions of the brain are broader and more closely approximated. 

 The epiphysis never breaks through the roof of the skull in 

 Cyclostomes, as it does in many Fishes. 



Elasmobranchii. The brain of these Fishes, like that of 

 Cyclostomes, is of a specialised form, characteristic of, and confined 

 to, the group, though the particular regions are much more highly 

 developed than in the Cyclostomi. According to their external form 

 two main types of Elasmobranch brains can bs distinguished. One 

 of these, seen in Spinax, Scymnus, and Notidanus, is character- 

 ised by its very narrow and elongated form, while in the rest of the 

 Elasmobranchii the individual parts are more closely compressed 



