THE BRAIN OR ENCEPHALON 



857 



side, the development of a groove or furrow, the primary arcuate fissure, which 

 demarcates the olfactory region (rhinencefkalon) into a cephalic and a caudal 

 portion. The cephalic portion develops into 

 a blind tubular diverticulum, which grows 

 cephalad to form the olfactory bulb and tract, 

 its central cavity becoming obliterated (persist- 

 ent in certain other mammals), while the caudal 

 portion forms the roots of the olfactory nerves, 

 the anterior perforated space or preperforatum and 

 the subcallosal gyre. 



(r/) At the ventral margin of the hemicerebral 

 or secondary fore-brain vesicle an excessive 

 proliferation of cells results in the production of 

 several ganglionic masses the basal ganglia, 

 of which the largest are the lenticular nucleus 

 (Icnticula) and caudate nucleus (caudatum). 



(e) The median cephalic terminal wall inter- 

 vening between the large hemicerebral vesicles 

 persists as a thin and relatively undeveloped 

 lamina, the lamina terminalis. 



(f) The remainder of the fore-brain undergoes 

 great hypertrophy in its lateral walls to form the 

 optic thalami, while the ventral portion develops 

 moderately to form the hypothalamus, tuber cine- 

 reum, posterior lobe of pituitary body, and corpora 

 albicantia. The dorsal wall fails to develop, 

 and remains epithelial except at a point imme- 

 diately adjacent to the quadrigeminal lamina of 

 the mid-brain; here a diverticulum grows out 

 to form the pineal body or epiphysis (a rudi- 

 mentary structure in man, but undoubtedly of 

 functional use in ancestral vertebrates). 



(gr) The cavity of the primary fore-brain vesicle 

 undergoes alterations in form as the secondary FlG 624 ._ Plan showing the mode of 



metamorphoses of itS Walls proceed in the COUrse formation of the ventricles of the brain 



* i 11 ii i an " * ne central canal of the spinal cord: 



OI development. Ihe hollow Cerebral buds SO .4 . Prosencephalon. B. Thalamencephalon. 



. ,, . ii ,1 n ,1 i ,1 C. Mesencephalon. D. Metencephalon. 



rapidly Outstrip all Other parts Ot the brain that E. Myelencephalon. F. Central canal of 



.i , i ,1 , . i . . , cord. G. Lateral ventricle. H. Foramen 



their internal cavities, the lateral ventricles, O f Monro. (After Gerrish.) 



Somatopleure 



. H Notochord Aorta 



Pleuropen- 



toneal cavity 



Splanchnoplenre 



Omphalo- 

 mesenteric vein 



FIG. 625. Transverse section of a portion of a chick embryo of twenty-nine hours incubation. 

 (From Duval's Atlas d'Embryologie.) 



become the most spacious of the ventricular system. The great hypertrophy of 

 the thalamic ganglia in the lateral walls of the primary fore-brain determine the 



