650 



each side converge into the walls of the interventricular foramen and 

 in larvae with wide foramina and in 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 hemispheres in the form 

 shown by Figs. 1 and 2, the zona limitans lateralis representing the 

 locus of the sulcus diencephalicus 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 olfactory bulb was undoubtedly the site of the initial telen- 

 cephalic evagination, but afterwards secondary olfactory tissue and 

 correlation tissue of all four laminae of the rostral end of the neural 

 tube were involved in this evagination and then further diff"erentiated 

 in situ. 



These conclusions may be recapitulated as follows: 



An examination of the brains of adult amphibians in comparison 

 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 fundamental 

 longitudinal divisions of the early neural tube as defined 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 thalami and hypothalamus. These are functionally defined 

 and are structurally evident in vertebrate embryos generally and in 

 all adult Amphibia. 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 in the adults of Amphibia, each cerebral 

 hemisphere is naturally divided into four parts which correspond re- 

 spectively 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 Am- 

 phibia are shown in the diagrams of Figs. 1 and 2. 



The olfactory bulb occupies the terminal part of the hemisphere 



