THE BRAIN OR ENCEPHALON 855 



stages and the disposition of certain of the cranial nerves afford a clue to the defi- 

 nitive segmentation of the brain. According to the most recent researches, as 

 many as eleven, sixteen, and even more neuromeres have been established in 

 various vertebrate brains. The hind-brain alone shows from six to eight such 

 neural segments (Figs. 620 and 621). The whole matter is yet so obscure that 

 confusion will be avoided by restricting our description to the three primary 

 divisions and their derivatives without insisting upon the recognition of further 

 definitive segments proposed by various authors in consequence of preconceived 

 ideas obtained from the complicated adult structure of the brain. At this transi- 

 tional period the student is, however, obliged to be familiar with the commonly 

 accepted yet provisional schemes of segmentation and a comparative view 

 is given in the following table: 



TABLE SHOWING COMPARISON OF THE SEGMENTAL SCHEMAS ADOPTED BY 



The Anatomische Gesellschaft The Association of American 



in 1895. Anatomists in 1897. 



I. RHIXEXCEPHALOX. 

 Paries ventrales. Partes dorsales. Bulb; o , factorii with their tractg; 



VI. TELEXCEPH^LOX P art - f the aula and of the P re ' 



commissure. 



Cor P u * rhinen- 



Pars optica hypothalami. 



(. cephalon; palli 



ium. II. PROSENCEPHALOX. 



Palliums, connected by part of the 

 aula and part of the precommis- 

 l sure. 



V. DlEXCEPHALOX. 



III. DlENCEPHALOX. 



Pars mammilaris hypothalami. { */^;* a ~ f Thalami, including the chiasm; 



geniculate bodies. 



IV . MESEXCEPHALOX. 



Pedunculi cerebri. Corpora quadrigemina. 



IV. MESEXCEPHALOX. 



III. ISTHMUS RHOMBEXCEPHALI. 



Crura and quadngemmum. 



Pedunculi cerebri. / B , rachia conjunctiva; ve- 



t him medullare antenus. 



II. METEXCEPHALOX. ^ V. EPEXCEPHALOX. 



Pons. Cerebellum. /Cerebellum; pons; preoblongata. 



I. MYELEXCEPHALOX. ") VI. METEXCEPHALOX. 



Medulla oblongata. ) Postoblongata. 



Brief Consideration of the Phases of Development of the Brain Tube, 

 I. Fore-brain. The cephalic or fore-brain vesicle widens and expands most 

 rapidly and attains to a comparatively large size even before the mid- and hind- 

 brain vesicles become markedly defined. A series of remarkable developmental 

 changes ensue in the following order: (a) Optic vesicles appear as two diverticula, 

 each budding from either side of the primary fore-brain vesicle, their distal ends 

 growing considerably and coming into contact with the overlying epidermis, 

 while the proximal or attached ends assume a stalk-like shape. The distal 

 sac-like end becomes invaginated and forms the retina of the eye, while the 

 stalk upon obliteration of its cavity forms the optic nerve. It is necessary to 

 state here that as development proceeds the optic stalks become relatively shifted 

 caudad and form more intimate connections with the thalamus and mid-brain. 



(6) A second pair of budding vesicles arises cephalad in the dorsal portion of 

 the fore-brain vesicle and is destined to develop into the ponderous cerebral hemi- 



