THE NERVOUS SYSTEM 



543 



Mesencephalon 



Shombencephalon 

 Myelencephalon 



Amnion (cut) 



Mesodermal segment 14 



Open neural groove 



Prosencephalon 



Stomodaum 



Amnion {cut) 



Yolk sac 



Body stalk 



Fig. 448. — Human embryo of 2.4 mm. showing the neural tube partially closed. 

 (From Ranson's "The Anatomy of the Nervous System," after KoUmann; courtesy of 

 W. B. Saunders Company.) 



CEPHALIC FLEXURE 

 OlENCEPHALON 

 TELENCEPHALON 

 I.FOREBRAIN 



OPTIC VESICLE ^_ jf, 

 OLFACTORY LOBE-^*^ 

 II.MIDBRAIN INFUNDIBULUM 



MAMMILLARYBODY 

 OTIC CAPSULE 



III.HINDBRAIN 



CEREBELLUM 



MESENCEPHALON 



EPIPHYSIS 

 DIENCEPHALON 

 TELENCEPHALON 

 METENCEPHAUON 



YELENCEPHALON 



OLFACTORY LOBE 



PONTINE FLEXURE 

 CERVICAL FLEXURE 



SPINAL CORD 



A. B, C. 



Fig. 449. — 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-week embryo in lateral 

 aspect. (Redrawn after His and Hardesty.) 



