MORPHOLOGY OF THE FOREBRAIN 477 



taken transversely through the diencephalon. The morpholog- 

 ical transverse section here is inclined to the long axis of the brain 

 as indicated by the line A-B of fig. 22. The chief features of such 

 a section through the rostral end of the diencephalon of both 

 Urodela and Anura are indicated in fig. 83, which shows that the 

 walls of the neural tube here are composed of ten longitudinal 

 columns or laminae. Besides the unpaired membranous roof 

 plate and floor plate, there are four ridges on each side, the epi- 

 thalamus, the pars dorsalis thalami, the pars ventralis thalami and 

 the hypothalamus, separated by the dorsal, median and ventral 

 sulci of the diencephalon. 



The mode of development in early embryos shows that the eight 

 massive columns are not produced by a passive plication of the 

 walls due to extrinsic forces; but that each column is a center of 

 more active proliferation of neuroblasts than the intervening 

 sulci, and has, therefore, doubtless been differentiated under the 

 influence of definite functional requirements. 



The roof plate and floor plate converge into the lamina termi- 

 nalis, where of course they end. The four massive columns on 

 each side converge into the interventricular foramen, and in larvae 

 with wide foramina and adult urodeles they may be followed 

 through the foramina into the evaginated hemispheres. Bearing 

 in mind the fact that during development the roof plate and floor 

 plate retain permanently their primitive attachments to the 

 lamina terminalis, and that it is only the massive lateral columns 

 which are evaginated into the hemispheres, it clearly follows that 

 these columns of the diencephalon are continued into the hemis- 

 pheres in the form shown by the accompanying diagram (fig. 

 84), the zona limitans lateralis representing the locus of the sulcus 

 medius and the zona limitans medialis the line of union of the 

 dorsal and ventral columns in the lateral evaginations rostral to 

 the fusion of the roof plate and floor plate in the lamina terminalis. 



The massive dorsal columns 1 and 2 are interrupted by the 

 di-telencephalic fissure and the telencephalic portions are fully 

 evaginated into the hemispheres. In the cyclostomes column 2 

 is evaginated to form the area olfactoria of Johnston, while column 

 1 remains unevaginated in the telencephalon medium as the 



THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO. 5. 



