CORRELATING CENTERS IN THE DIENCEPHALON. 271 



the lateral border of the nucleus of the tractus strio-thalamicus, 

 which belongs to the thalamus. The hypothalamic structure 

 extends up lateral to this nucleus, however, upon the lateral surface 

 of the thalamus and midbrain nearly to the ventral border of the 

 tectum mesencephali (Figs. 133, 134, 135). This area is quite 

 free from myelinated fiber tracts but is traversed by the myriads 

 of fine fibers of the tractus lobo-bulbaris, to which it contributes 

 additional fibers. The dorsal border of this area is defined by a 

 dense bundle of this tract. In bony fishes the typical hypothal- 

 amic structure extends up in precisely the same way upon the 

 lateral surface of the thalamus and midbrain and on account of 

 the great size of the tectum mesencephali the base of the latter 

 is crowded down into contact with this portion of thehypothal- 

 amus. In both the sturgeon and the bony fish the hypothalamic 

 structure. is sharply limited dorsally by a groove and a septum of 

 connective tissue extending deep into the brain substance. The 

 limit between the thalamus and hypothalamus (Fig. 134) runs 

 from a point near the base of the tectum mesencephali to the 

 lateral border of the nucleus of the tractus strio-thalamicus. 

 The structure of the hypothalamus is uniform throughout and is 

 remarkably constant in the series of vertebrates. The fiber tracts 

 connected with the hypothalamus are quite different from those 

 connected with the thalamus. To what extent the structure of 

 the hypothalamus extends up on the lateral surface of the thala- 

 mus in other classes than ganoids and teleosts is not known. 



Fortunately we have an account of the genetic relations of this 

 hypothalamic structure which appears so anomalous in the adult 

 brain. In early teleost embryos the brain consists of a number 

 of ring-shaped neuromeres which have been described in Chapter 

 III. As indicated in that chapter, the first neuromere gives 

 rise to the forebrain, the second gives rise to the retina and comes 

 to have the optic chiasma at its ventro-cephalic border (Chap. 

 VIII). The third neuromere forms the thalamus, nucleus haben- 

 ulae and epiphysis. The fourth and fifth neuromeres form the 

 mesencephalon. One of the earliest differentiations of form to 

 appear in the brain segments of bony fishes is an expansion of 

 the ventral wall of the second neuromere, which begins to crowd 



