288 H. SAXTON BURR 
its fibers are ascending to the hemisphere from the thalamus. 
No other defined group of fibers is evident at this stage. Only 
a very slight differentiation into the pumary and secondary 
nuclei may be noted. 
From this period forward development is but an elaboration of 
the processes so far described. At stage 40 the entire hemi- 
sphere is completely defined. The forward growth of the dorso- 
anterior pole of the hemisphere has progressed with the formation 
of the definitive lateral ventricles and septum ependymale. The 
velum transversum is clearly defined joining the pars ventralis 
thalami of one side with that of the other. The lateral swing of 
the dorsal margin of the hemisphere has progressed and is accom- 
panied by a slight increase in the evagination of the ventral 
margin. 
The olfactory bulb may be seen clearly at this stage, occupying 
the ventro-lateral region of the anterior pole of the hemisphere. 
The ventro-median region of the pole is occupied by the nucleus 
olfactorius anterior. Running caudad from the bulb there may 
be distinguished in the ventro-lateral region of the hemisphere 
the tractus olfactorius ventro-lateralis incompletely separated 
from the lateral forebrain tract which lies immediately dorsal 
to it (fig. 23). The tract tends to disappear caudally in the region 
of the primitive corpus striatum which occupies the nuclear 
region of the hemisphere wall at the level of the posterior free 
margin of the velum transversum. Slightly dorsal to the lateral 
forebrain tract, the tractus olfactorius dorso-lateralis component 
appears as a thin lamina of fibers starting in the olfactory bulb 
and running caudad to disappear in the posterior pole. The pars 
dorso-medialis at this stage is a relatively thin layer of cells dis- 
appearing caudally into the ventral lamina of the velum trans- 
versum and thickening anteriorly as it becomes continuous over 
the anterior pole with the anterior olfactory nucleus. The sep- 
tum ependymale is entirely membranous except where it becomes 
continuous with the medial wall of the hemisphere at the level 
of the olfactory bulb. A very small group of fibers forming a 
ventral expansion of the white matter of the pars ventro-lateralis 
may be traced from the olfactory bulb as far caudally as the 
