498 C. JUDSON HETtRICK 



SUMMARY 



An examination of the brains of adult amphibians in compari- 

 son with embryos of these animals and of reptiles and mammals 

 reveals a simple morphological pattern which is common to the 

 diencephalon and the telencephalon and which rests directly upon 

 the funadmental longitudinal divisions of the early neural tube 

 as denned by His. In the diencephalon the six primary laminae 

 of His (roof-plate, floor plate, dorsal plates and ventral plates) 

 become ten by the division of the dorsal and ventral laminae on 

 each side into two parts, so as to give in addition to the unpaired 

 membranous roof plate and floor plate four others on each side, 

 viz., the epithalamus, pars dorsalis thalami, pars ventralis thai- 

 ami and hypothalamus. These are functionally defined and are 

 structurally evident in vertebrate embryos general^ and in all 

 adult Amphibia (see fig. 22 and pp. 466 ff.). 



In the telencephalon the roof plate and the floor plate converge 

 in the lamina terminalis and the massive side walls are more or 

 less completely evaginated to form the cerebral hemispheres. 

 In embryos generally and more clearfy in the adults of Amphibia 

 each cerebral hemisphere is naturally divided into four parts 

 which correspond respectively with the four primary laminae of 

 the lateral wall of the neural tube whose evagination produced 

 the hemisphere. The relations of these parts of the telencephalon 

 and diencephalon in adult Amphibia are shown in the diagrams 

 of figs. 83 and 84 (see p. 477). 



The olfactory bulb occupies the terminal part of the hemisphere 

 (not of the -primary neural axis) and the other four parts of the 

 hemisphere are so related that the two ventral parts correspond 

 with and pass backward directly into the ventral or motor lamina 

 of the lower parts of the neural tube, the visceral efferent functions 

 predominating in the ventro-median part and the somatic effer- 

 ent in the ventro-lateral part. The two dorsal parts of the hemi- 

 sphere correspond with the dorsal or sensory lamina of the neural 

 tube, but direct continuity between the telencephalic and dien- 

 cephalic segments of the dorsal lamina is interrupted in forms 

 above the fishes by the great di-telencephalic fissure and (in 

 Amniota) the posterior chorioidal fold. 



