542 



COMPARATIVE ANATOMY 



appear, corresponding with forebrain, midbrain, and hindbrain, the fore- 

 brain differing from the other two in being anterior to the notochord. 

 From these, by processes of local unequal growth, all the parts of the 

 definitive brain are differentiated. Experiments demonstrate that the 

 position of the three brain divisions is predetermined in the open neural 

 plate. Soon after their closure and expansion as midbrain and hindbrain 

 vesicles, the lateral walls become divided by a longitudinal sulcus into a 

 ventral basal plate and a dorsal alar plate. Less clearly seen is a narrow 

 floor plate in the mid ventral line, and a roof plate in the mid-dorsal line. 



= -" . 1a"l>r pl>te 



ibasal plate 



ifloor plate 



- ' MESEINCE PHAL ON 



EPIPHYSIS 



TELENCEPHALON 



METENCEPHALON 



MYELENCEPHALON 



TERMINALIS. 



PREOPTIC RECESS 



Fig. 447. — Diagrams of the development of the brain. A, Early neural plate before 

 closure, with zones marked; B, Longitudinal section of early brain tube; C, Later stage 

 with parts diiTerentiated. The dorsal zone (alar plate) is finely stippled; the ventral 

 zone (basal plate) is coarsely stippled; the floor plate is cross-hatched. Cer., cere- 

 bellum; c.str., corpus striatum; inf., infundibulum; mam., mammillary recess; olf., 

 olfactory lobe; pal., pallium; thai., thalamus. (After Kingsbury modified.) 



Since this sulcus does not develop in the forebrain, and ends where the 

 notochord ends, it appears that the forebrain consists of alar and roof 

 plates only. By the time a human embryo is a month old, the primitive 

 forebrain vesicle has begun to divide into the anterior telencephalon 

 and posterior diencephalon. In a five weeks embryo, the hindbrain has 

 begun to divide into the anterior metencephalon and posterior myelen- 

 cephalon. The undivided midbrain is the mesencephalon. These five 

 brain regions occur in all vertebrates, and from them all the parts of the 

 adult brain are formed. 



Brain Flexures. While the subdivision of the primary vesicles is 

 taking place in ontogenesis, the brain undergoes in amniote embryos three 

 successive flexures, the cephalic or primary, the pontine, and nuchal or 

 cervical. The cephalic flexure occurs in the midbrain region, the other 

 two in the region of the hindbrain. All three flexures are in a vertical 

 plane, but the bend of the pontine is the reverse of the other two. The 



