272 NERVOUS SYSTEM OF VERTEBRATES. 



back beneath the third neuromere (Fig. 19). This ventro- caudal 

 projection from the second neuromere is the beginning of the 

 hypothalamus. Its early appearance in the embryo is due both 

 to its importance and to the fact that it is phylogenetically an old 

 structure. The hypothalamic rudiment continues to expand until 

 it forms the large inferior lobes beneath the third neuromere. 



i> 



Tr. bulbo- 



tectalis . 



Tr. haben.- 



pedunc. 



Nuc. of fasc. 

 long. med. 



"<-- 





>^^| 

 ^^i 



Tr. tecto- 

 cerebel. 

 Tr. lobo- 

 bulbaris 



■Comm. post. 



Tr. strio-thalar 



Tr. lobo-epistr 

 — =" aticus 



/ Tr. thal.-saccus 



End nucleus of Tr. sacco-thalam. 



Fig. 134. — Transverse section through the inferior lobes of the sturgeon. 



The ventricle of the second neuromere is carried back wath the 

 outgrowth and fuses with that of the third neuromere along the 

 Hne meso-ventral to the tractus strio-thalamicus and its nucleus. 



The diencephalon therefore consists of the third neuromere 

 and a part of the second, the remainder of the second being carried 



