647 



Separation of the medial and lateral components of the basal forebrai» 

 tract. Nevertheless the morphological pattern is the same. I am 

 satisfied from the examination of an extensive series of embryonic and 

 adult vertebrate brains that the Anura exhibit a simple structural 

 pattern of cerebral hemisphere which reveals with great clearness the 

 underlying functional characteristics of the telencephalon in all verte- 

 brates which possess fully evaginated hemispheres. 



The analysis of the amphibian diencephalon is equally simple and 

 it also sheds light upon certain functional relationships. Fig. 2 illustrates 

 the form of the diencephalon as seen in transverse section of either 



LF.T 



n?.T 



Fig. 1. 



Fig. 1. Diagrammatic cross section through the cerebral hemispheres in front of 

 the lamina terminalis of the frog. 



A dorsal angle of hemisphere. B zona limitans lateralis and fissura endo-rhinalis. 

 C ventral angle of hemisphere. D- V zona limitans medialis and fissura limitans hippo- 

 campi. L.F.T. lateral forebrain tract. 3I.F.T. medial forebrain tract. 



Fig. 2. Diagrammatic cross section through the diencephalon of Urodela and 

 Anura. On account of the diencephalic flexure, the section must be taken obliquely to 

 the long axis of the brain in the plane A — B of Fig. 3, in order to pass transverse to 

 the thalamic axis. The numbers 1, 2, 8, and 4 and the letters A, B, and C, mark 

 corresponding structures in Figs. 1 and 2, the two figures being designed to illustrate 

 the way in which the cerebral hemispheres have been formed by the lateral evagination 

 of the walls of the neural tube ; see the text. 



A sulcus diencephalicus dorsalis. B sulcus diencephalicus medius. C sulcus di- 

 encephalicus ventralis. D roof plate. V floor plate. 



Urodela or Anura. Epithalamus and hypothalamus are separated from 

 the thalamus by clearly defined ependymal sulci and the thalamus is 

 divided into dorsal and ventral parts by a third sulcus which is con- 

 tinued backward into the sulcus limitans of the midbrain. 



Fig. 3 shows the relations of the diencephalic sulci as recon- 

 structed from sagittal sections of the brain of adult Amblystoma 

 tigrinum. The sulcus limitans is clearly preserved for its entire lengthy 



