THE THALAMUS 129 



vertebrate. The manifold diversities of cerebral or- 

 ganization presented by the various species of verte- 

 brates have all been derived from this simple form by 

 unequal growth and internal differentiation of the 

 walls of this neural tube. Many factors have co-op- 

 erated, chief of which is the type of functional inter- 

 connection of nerve centers demanded by the action 

 systems of the several species. 



In the forebrain a very early stage in this history 

 is shown in the brain of the sturgeon (Fig. 28). In 

 this fish the parts of the primitive neural tube into 

 which the olfactory nerves enter are thickened and 

 ballooned outward or evaginated to form the olfac- 

 tory bulbs, and the contiguous parts of the lateral 

 walls are locally thickened without evagination to 

 form the primitive endbrain (telencephalon medium). 

 The structural plan of this brain is very simple. 

 The thickenings which form the primitive endbrain 

 contain the nerve centers within which olfactory in- 

 fluences coming in from in front are correlated with 

 non-olfactory sensory impulses coming in from be- 

 hind, in conformity with the behavior pattern of this 

 fish. 



The thalamus is the narrowest part of this brain, 

 and the fibers which ascend from it into the endbrain 

 are few in number. A view of the under surface of this 

 brain would show a much larger hypothalamus. 

 These two parts, together with a small epithalamus, 

 make up the betweenbrain or diencephalon in this 



