230 C. JUDSON HEKRICK 



and the consequent position of the habenula relatively farther 

 forward in these forms. In general, the sulcus subhabenularis 

 forms the ventral boundary of the habenula and the sulcus 

 dorsalis thalami that of the posthabenular region when the 

 latter is extensive. 



The posthabenular region (synencephalon of Kupffer, '06) 

 has been well named by Gaupp ('99, p. 70) pars intercalaris 

 diencephali, and the commissure contained within it ('99, p. 92) 

 the commissura tecii diencephali. In the early embryos of 

 many vertebrates the pars intercalaris is large; but its fate in 

 most types other than the Amphibia is obscure. 



The pars dorsalis thalami is relatively small in Necturus and 

 in urodeles generally. It is larger in the frog and in Amniota 

 it is still larger, comprising the great sensory nuclei of the thala- 

 mus (nucleus sensitivus of Ram6n y Cajal). The pars ventralis 

 thalami exhibits the converse evolutionary history, diminishing 

 from its relatively large proportions in Amphibia to insignifi- 

 cance in higher forms. 



At the rostral end of the thalamus there is a strong eminence 

 which projects into the ventricle immediately behind the inter- 

 ventricular foramen and is termed the eminentia thalami (figs. 

 41, 42, 43, 48, 63, 64, em.ih.). Lying somewhat dorsad and 

 caudad of the eminentia thalami is a much smaller and less 

 prominent eminence which marks the rostral end of the pars 

 dorsalis thalami. As we shall see beyond (p. 244), this is the 

 nucleus of the pars optica thalami (figs. 42 to 45, 48, 63, 64, 65, 

 68, nuc.p.o.th.). Laterally of this in the white layer is a super- 

 ficial area of characteristic dense neuropil which in some prep- 

 arations produces a slight eminence on the pial surface of the 

 thalamus; it is here termed the pars optica thalami, for here 

 are the actual synapses between fibers of the optic tract and 

 thalamic neurons (fig. 1, 2, 25, 26, 40, 42 to 45, 48, 54, 56, 57, 

 p.o.th.). 



The hypothalamus and chiasma ridge are separated from 

 the preoptic nucleus by an oblique ventricular sulcus which 

 runs forward and ventralward in front of the chiasma ridge to 

 end in the deep preoptic recess (figs. 63, 64). This I interpret 



