144 



Basic Structure of Vertebrates 



cavity of the embryonic neural tube. In the arrangement of cranial 

 and spinal nerves, connected with brain and cord respectively, is 

 seen a segmentation corresponding to that of the body-muscles. 



Both notochord and neural tube become involved in development 

 of the vertebral column. The usually solid bodies or centra of the 

 vertebrae are formed around the notochord. Each centrum bears 

 dorsally a skeletal arch, the neural arch, which encloses the spinal 

 cord (Fig. 100). 



The dorsal location and the central cavity are the especially 

 unique features of the vertebrate central nervous organs as compared 

 to those of invertebrates. 



BRAIN 



In all vertebrates the wider cephalic region of the neural tube early 

 produces three enlargements (Fig. 135) known as primary brain- 

 vesicles and indicated by number beginning anteriorly, or called 

 forebrain, midbrain and hindbrain. Meanwhile, the olfactory 

 sacs are developing just in front of the brain, the otic sacs at each 

 side of the rear of the brain, and an eye between the olfactory and otic 

 sacs of each side (Fig. 115). The primary nervous connections of the 

 olfactory organs are with the forebrain (i.e., with parts of the adult 

 brain which are derived from the embryonic forebrain) ; the optic 

 nerves become connected with the midbrain ; and the nerves to the ear 

 proceed from the hindbrain. 



In the course of the later development of the brain, the embryonic 

 forebrain is differentiated into two well-defined parts of the adult 

 brain; the midbrain remains undivided; and the hindbrain forms two 

 distinct regions. These five regions are clearly recognizable in all adult 



CEPHALIC FLEXURE 

 DIENCEPHALON 

 TELENCEPHALON. 

 FOREBRAIN 



OPTIC VESICLE 

 OLFACTORY LOBE 

 II MIDBRAIN INFUND 



CEREBELLUM 



MESENCEPHALON 



EPIPHYSIS 

 DIENCEPHALON- 

 TELENCEPHALON 

 METENCEPHALON « 



TELENCEPHALON 



OLFACTORY I 



PONTINE FLEXURE 

 CERVICAL FLEXURE 



SPINAL CORD 



Fig. 135. Three stages in the development of the human brain, showing the 

 brain-vesicles and flexures. (A) Early stage, dorsal aspect. (B) Brain of a three- 

 week embryo in lateral aspect. (C) Brain of an eight-week embryo in lateral aspect. 

 (After His and Hardesty. Courtesy, Neal and Band: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



