MORPHOLOGY OF THE FOREBRAIN 427 



less occurs in Amblystoma also, though I have not been able to 

 demonstrate it. Some of its fibers cross in the commissure, but 

 most of them ascend by way of the stria medullaris to the habenula 

 of the same side. It is therefore a component (partly decussating?) 

 of the tractus olfacto-habenularis. I have not been able to find 

 the "tractus lobo-epistriaticus" described by Johnston ('06, p. 

 309) as passing from the hypothalamus to the primordium hippo- 

 campi by way of a decussation in the commissura pallii anterior. 

 These fibers may be the same as the ones which I interpret as 

 tractus cortico-thalamicus, or I may have overlooked them. 



On the basis of the relations just described and of the study of 

 an extensive series of brains of Necturus and the frog I interpret 

 the composition of the fimbria and stria medullaris systems of 

 fibers in urodeles as expressed in the following summary (see also 

 fig. 22). 



1. The fimbria complex at its rostral end receives fibers of the 

 tractus olfactorius dorso-medialis, is connected by association 

 fibers (ascending and descending) with the nucleus medianus 

 septi and gives rise to the columna fornicis. At its caudal end 

 it contains fibers of the commissura pallii anterior, tractus cor- 

 tico-habenularis medialis and tractus cortico-thalamicus. The 

 hippocampal commissure is divided into two parts, the commis- 

 sura pallii anterior and posterior. 



2. The commissura pallii anterior includes commissural fibers 

 between the dorso-medial parts of the hemispheres and decus- 

 sating fibers of the tractus cortico-habenularis medialis and cor- 

 tico-thalamicus. 



3. The stria medullaris is a very complex tract bordering the 

 taenia thalamia. It includes a part of the course of all of the 

 following tracts. 



4. The commissura pallii posterior. These are unmedullated 

 fibers which arise from the ventral surface of the posterior pole 

 of the hemisphere and pass directly medial-ward to join the stria 

 medullaris, within which they ascend to the commissura superior 

 and thence pass to the posterior pole of the other hemisphere. 

 They are homologous with the similar tract described by Elliot 

 Smith ('03) in reptiles under the name, commissura aberrans. 



