DIENCEPHALON >;{9 



ropil. I have calleci tliis c'oiniiu)ii jxk)! of unditt'orentiated tissue of llie 

 dorsal thalainiis and dorsal part of the ventral thalamus tlie "genieu- 

 late neuropil," on the assumption that out of it the medial and lateral 

 geniculate bodies have emerged (]). '-I'-U). In the course of i)hylogeny 

 the more sharp segregation of the optic terminals was accompanied 

 by the differentiation of the lateral geniculate body, a process which 

 is well advanced in the frog ('"25), and the differentiation of the 

 cochlear apparatus and lateral lemniscus was accompanied by the 

 emergence of the medial geniculate body from the same common 

 pool. This hypothesis is held subject to revision, pending further 

 study of the related species. 



VENTRAL THALAMUS 



On the ventricular surface (figs. 1, '2, 15>, 95) the ventral thalanuis 

 is sharply delimited by the sulcus medius thalami (.s-.m.) above and 

 the sulcus ventralis (s.r.) below, and it is separated into anterior and 

 posterior parts by a depressed area containing in some si)ecimens a 

 shallow sulcus. The two parts differ in embryological origin and con- 

 nections, yet in the adult brain their most fundamental features are 

 similar ('42, p. 207). 



The posterior part of this field extends forward from the peduncle, 

 and in early functional stages it has similar structure and connec- 

 tions, though it is clearly in diencephalic territory. In these stages 

 the ])rimordium of the anterior i)art (area 7a of my analysis, ','??, 

 p. 393) lies at the di-telencephalic junction ('38, p. 213 and fig. 18), 

 and in prefunctional stages it is joined with area 7, which becomes 

 the corpus striatum. This anterior part of the ventral thalamus may 

 be genetically telencephalic, depending on how the arbitrary di- 

 telencephalic boundary is defined. 



In the adult ventral thalamus none of the nuclei of more highly 

 differentiated brains are well defined, though some local differentia- 

 tion is evident. Anteriorly, the eminentia thalami (figs. 2, 16, cni.fh.) 

 belongs in a series of bed-nuclei related with important tracts of 

 fibers at the di-telencephalic junction (chap, xviii). Both this area 

 and the underlying nucleus of the olfacto-habenular tracts discharge 

 fibers backward into the unspecialized gray of the anterior part of the 

 ventral thalamus (fig. 17). Between the anterior part of the ventral 

 thalamus and the dorsal (mamillary) part of the hypothalanuis there 

 are fibers passing in both directions which seem to be precursors of 

 the mammalian tr. mamillo-thalamicus ('396, p. 554 and figs. 22, 35). 



The posterior part has a thicker gray layer, which produces a 



