MORPHOLOGY OF THE FOREBRAIN 421 



The following description is based upon Amblystoma larvae 

 between 30 mm. and 40 mm. in length, specimens taken at the 

 time of metamorphosis and adults. 



In all of these specimens the olfactory bulb (consisting of the 

 glomeruli, mitral cells and granule cells) is confined to the lat- 

 eral and extreme rostral parts of the hemisphere. In reading a 

 series of cross-sections backward, as soon as the lateral ventricle 

 appears the olfactory bulb is found to lie wholly laterally of it; 

 but rostral to this level the layer of granule cells borders the whole 

 median surface of the section. This short region where the gran- 

 ular layers of the two hemispheres are closely approximated 

 (fig. 8), marks the site of the interbulbar union of the anuran 

 brain. This figure from an adult brain shows medullated and 

 unmedullated fibers from the mitral cells passing through the gran- 

 ular layer to accumulate on the median border of the hemi- 

 sphere (fig. 9). Fibers of the tractus olfactorius medialis at the 

 levels of these figures come from both the dorsal and ventral 

 parts of the olfactory bulb, but farther caudad from the ventral 

 part only. 



In the adult at the rostral end of the olfactory bulb the mitral 

 cells are separated from the glomeruli by a wide molecular layer. 

 There are clusters of cells among the glomeruli which correspond 

 with the subglomerular cells of Rubaschkin's description ('03) 

 and probably with the periglomerular neurones of Cajal. Far- 

 ther caudad the mitral cell layer is less compact and its cells 

 spread throughout the molecular layer. 



Beginning at the rostral tip of the lateral ventricle (fig. 9) the 

 median wall of the hemisphere is occupied by an extensive and 

 undifferentiated secondary olfactory nucleus, which I term the 

 nucleus olfactorius anterior, and which as we pass caudad spreads 

 through the medial and dorsal walls (fig. 10), the olfactory bulb 

 occupying the whole ventro-lateral wall. Medullated and unmed- 

 ullated secondary olfactory fibers pass from the olfactory bulb 

 into the dorsal border of the anterior olfactory nucleus and continue 

 caudad in this relation as tractus olfactorius dorso-lateralis. In a 

 similar way medullated and unmedullated fibers curve around the 

 ventral angle of the lateral ventricle to form the dorsal and ven- 



