BRAIN 



the foramen of Monro. The roof of the prosencephala becomes 

 the pallium which acquires such an enormous development in the 

 cerebral hemispheres of the highest Vertebrates. From the olfactory 

 lobes issue the nerves to the olfactory epithelium. The great 

 modifications in the shape arid relative size of the different parts of 

 the brain in the Craniata are, of course, the outward manifestations 

 of the differentiation and orderly arrangement of the ganglion cells 

 and nerve-fibres into an elaborate system of ' tracts ' and ' nuclei/ 

 which cannot be described in this volume. (Brain of Fishes : 

 Burne [76], Burckhardt [70], Studntfka [429], Johnston [249].) 

 A new paired cranial nerve of doubtful significance has recently 



to. 



Fio. 10. 



Longitudinal median section through the brain of an embryo fyrinax nlger. (After von 

 Kuppfer, from Hertwig's Hanilbuch.) (.', cerebellum ; 

 rhiasma ; <, epiphysiw 

 gringlion and commissure 



/, lobus posterior ; .17, mesencephalon ; Ml, myelencephalon ; Mt, metencephalon ; p.c, plica 

 cerebelli posterior ; /., reeessus ppticus ; s, saccus infundibuli ; t, telencephalon ; (.0, tectum 

 o]>ticum ; f./>, tuberculum posterius ; v, valvula cerebelli posterior ; v.t, velum transversum. 



median section tnrougn tne oram 01 an einuryo &jnnax inger. ^Aiier von 

 fertwig's Hanilbuch.) (.', cerebellum ; c.p, posterior commissure ; e, optic 

 )hysis ; e', paraphysi.s ; /.?, rhombo-mesencephalic fissure ; g.h, habenular 

 nmissure ; hy, hypophysis ; K, cartilaginous basis cranii ; /, infundibulum ; 



l^een described in Elasmobranchs, Amia, and Protopterm : it is 

 the nervus terminalis, which issues from the fore-brain near the 

 olfactory nerve, bears a ganglion, and supplies the epithelium of the 

 nasal sac (Pinkus [329], Allis [10], Locy [284ft], Johnston [248a]). 

 It may represent the dorsal root of the most anterior cephalic 

 segment. 



Further evidence concerning the segmentation of the head in 

 Craniates may be gained from a study of the gill-slits and visceral 

 arches (Gegenbaur, Koltzoff, etc.). The slits were probably 

 primarily intersegmental ; but their relation to the somites is not 

 very close. They pierce the lateral plate of mesoderm to reach the 

 exterior, and as they enlarge they are pushed backwards so as to 



