384 



CHORD ATE ANATOMY 



bordered by neural folds. The anterior more widely expanded portion 

 forms the brain, and the narrower posterior portion the spinal cord. The 

 transition between the two is, however, in most animals, gradual rather 

 than abrupt. 



MID- 



HINDBRAIN 



Ialar pl>te 



ibasal plate '""' 



^_1__1 jfloor plate 



- •' — -» MESENCE PHAL ON 



EPIPHYSIS 



METENCEPHALOM 



NOTOCHORD 



MYELENCEPHALON 



TELENCEPHALON 



PREOPTIC RECE 



Fig. 339. — 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 differentiated. 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.) 



CEPHALIC FLEXURE 

 DIENCEPHALON 



TELENCEPHALON 

 I. FOREBRAIN ^ 



CEREBELLUM 



OPTIC VESICLE 

 OLFACTORY LOBE 



IKIFUNOIBULUM 

 MAMMILLARY BODY 

 OTIC CAPSULE 



MESENCEPHALON 



EPIPHYSIS 

 DIENCEPHALON 

 TELENCEPHALON 

 METENCEPHALON 



YELENCEPHALON 



OLFACTORY LOBE 



PONTINE FLEXURE 

 CERVICAL FLEXURE 



SPINAL CORD 



A. B. C. 



Fig. 340. — Three stages — A, B, and C — in the development of the human brain, 

 showing the brain vesicles and flexures. A is an early stage, dorsal aspect, B the brain 

 of a three-weeks embryo in lateral aspect. C that of an eight-weeks embryo in lateral 

 aspect. (Redrawn after His and Hardesty.) 



As the neural folds rise, they bend towards the median plane and 

 finally unite to form a neural tube with an anterior enlarged brain and a 

 posterior constricted spinal cord. The closure of the neural tube begins 

 in the neck region and proceeds craniad and caudad. But even before 

 the neural folds in the cephalic region unite, a series of three expansions 



