MORPHOLOGY OF THE FOREBRAIN 423 



The dorso-medial wall of the hemisphere from this point cau- 

 dad to the posterior pole has the same structure and fiber con- 

 nections as in the frog. Its cells are not arranged, as else- 

 where in the hemisphere, in the form of primitive ventricular 

 grey, but are scattered uniformly through the substance of the 

 wall. Though no true cortex is formed here, the comparative 

 anatomy of this part makes it plain that the cortex hippocampi 

 of Amniota is differentiated within this region. Accordingly it 

 is properly termed primordium hippocampi. From its rostral 

 end a few medullated fibers accumulate close to the medial sur- 

 face and descend to the ventro-medial angle, where they turn 

 caudad and accompany the ventral forebrain tract to the hypo- 

 thalamus (fig. 11). This tract is present also in adult Necturus 

 and here, though the number of medullated fibers is still less than 

 in Amblystoma, they run more separate from the other medul- 

 lated fibers in the ventral forebrain tract so that the whole course 

 can be read with ease. This is clearly the columna fornicis (see 

 fig. 22). Many unmedullated fibers accompany this medullated 

 tract between the primordium hippocampi and the pars ventro- 

 medialis hemisphaerii, but whether any of these extend to the 

 hypothalamus and thus form a part of the columna fornicis is 

 not clear. Similar unmedullated fibers are abundant between the 

 primordium and the nucleus medianus septi for the whole extent 

 of the latter. They are probably for the most part short associa- 

 tion fibers. There is also a thin superficial layer of association 

 fibers running around the dorsal angle of the hemisphere between 

 the lateral and medial parts. This extends the whole length of 

 the hemisphere. Accompanying the columna fornicis are many 

 unmedullated fibers and a few medullated fibers between the pars 

 ventro-medialis of the hemisphere and the hypothalamus, the 

 whole complex being the medial forebrain tract. 



Extending caudad from the olfactory bulb is the pars ventro- 

 lateralis hemisphaerii. This contains, in addition to the tractus 

 olfactorius ventro-lateralis already referred to, medullated and 

 unmedullated ascending and descending fibers between the hemi- 

 sphere and the thalamus, the lateral forebrain tract. 



A cross-section taken behind the olfactorv bulb of adult Am. 



