598 



VERTEBRATE LIFE AND ORGANIZATION 



Oplic 

 vesicle- 



Mebencephalon 



Mesencephalon 1 ,-~-^^^^ 



rForebrain 



-Midbrain 



rHindbrain 



Dien- 

 ccphalon 



My elen cephEjon 



Telencephalon 



— { — Spinal 



Heart 



Spinal cord 



—Diencephalic 

 outline 



Cerebral hemispherc" 

 (Tclencephcxlon) 



Olfactory bulb 

 (.Telencephalon) 



B 



cuius iMesencephsJic 



ferior 

 lliculus 



roof 



Cerebellum. 

 (Metencephalon) 



Pons 

 (Metencephalon) 



Medulla oblongata 

 (Myelencephalon) 



Spinal cord 



Figure 29.13. Three stages in the development of the human brain. A, The three 

 primary brain regions can be recognized in an embryo that is about three and one 

 half weeks old. B, All five brain regions are evident in an embryo seven weeks old. 

 C, The various structures foimd in a fully de\eloped brain are beginning to differentiate 

 in an embryo eleven weeks old. (After Patten.) 



The central canal of the spinal cord extends into the brain, and is 

 continuous with several large, interconnected chambers known as ven- 

 tricles (Fig. 29.14). A lateral ventricle lies in each cerebral hemisphere 

 and each is connected with the third ventricle in the diencephalon by a 

 foramen of Monro. The aqueduct of Sylvius extends from the third 

 ventricle through the mesencephalon to a fourth ventricle in the meten- 

 cephalon and medulla oblongata. All of these passages are filled with a 

 lymphlike cerebrospinal fluid, which is produced by vascular choroid 

 plexuses. Choroid plexuses develop in the thin roof of the diencephalon 

 and medulla and are also present in the lateral ventricles of mammals. 

 Cerebrospinal fluid escapes from the brain through foramina in the roof 

 of the medulla, and slowly circulates in the spaces between the layers of 



