STRUCTURE OF THE VERTEBRATES 



273 



Prosencephalon 



Mesencephalon 



Rhombencephalon 



fRhinencephalon (olfactory lobes) 

 Telencephalon (cerebrum and ventral 

 regions) 

 Diencephalon 



Optic lobes 



r IMetencephalon (cerebellum) 

 (^ Myelencephalon (medulla) 



As the brain develops in the early embryo flexures, or bends, 

 appear. They are more prominent in the higher vertebrates and 

 are retained throughout life, while in the lower vertebrates they 

 are weak in the embryo and almost entirely disappear in the 



A. Primary 

 (Apical) 



B. Secondary 

 (Pontal) 



Fig. 154. Flexures of the Brain (Chick), 



C. Tertiary 

 (Nuchal) 



adult. The first, primary, flexure occurs in the midbrain, and the 

 forebrain is bent ventrally until it lies at right angles to the 

 cord. The second flexure is at the end of the medulla {nuchal 

 flexure) and this bends the brain until the forebrain almost 

 touches the cord, its anterior end pointing posteriorly. The third 

 ipontal) flexure through the cerebellar region bends the brain 

 backward toward its original position. In the fish and am- 

 phibia the third flexure carries the brain back into line with the 

 antero-posterior axis of the body. In the amniotes the first and 

 second flexures are progressively stronger; and in the birds and 

 mammals the forebrain remains at right angles to the axis of 

 the body. 



The neurocoel is the cavity formed in the central nervous 

 system when the groove closes. It is a typical characteristic of 



