286 H. SAXTON BURR 
bulging. That portion of the wall lying ventral to the sulcus 
shows an oval-shaped thickening which reduces the ventricle at 
this point to a narrow slit. This oval area represents the portion 
of the wall of the neural tube from which will develop the hemi- 
sphere. It lies immediately contiguous to the olfactory placode, 
although no connection between the two has been established at 
this time. As has been shown elsewhere (Burr, ’16), this thick- 
ening is an inherited pattern of development, since it occurs 
regardless of the presence of the placode. 
As soon as the ingrowing olfactory fibers reach the hemisphere 
there is evident the first external sign of evagination. The ap- 
pearance of the neural tube at this point (stage 35, Harrison) is 
shown in figures 15, 16, and 20. Externally, the area of the 
lateral wall ventral to the bulging produced by the thalamus 
shows signs of its own outpouching. This outpouching is pro- 
duced by the rapid proliferation of the cells lying in the neural 
wall parallel to the sulcus interencephalicus anterior from the 
velum transversum posteriorly to the lamina terminalis 
anteriorly. 
The growth of the sheet of cells lying in this position swings 
the wall of the neural tube outward, deepening the external di- 
telencephalic groove and the s. interencephalicus internally. ‘The 
latter from this point on is established as the sulcus angulus dor- 
salis of the hemisphere. ‘There is still at this stage little change 
in the organization of the hemisphere wall, the radially arranged 
cells being scattered more or less evenly through it, though an 
area free of cells begins to appear at the surface of the neural 
tube. 
From this point on a definitive hemisphere may be recognized. 
The di-telencephalic groove is deepened externally by the bulging 
of the dorsal margin of the hemisphere. This outward swing of 
the dorsal lip of the evagination first clearly defines the hemi- 
sphere. It bears no apparent relation to the nasal placode, since 
it occurs dorsally and anteriorly to it. The region of most rapid 
growth seems to lie between the posterior limit of the velum 
transversum, defined at the earlier stage, and the lamina ter- 
minalis. This area maintains its growth supremacy throughout 
