EARLY DEVELOPMENT OF HEMISPHERES 287 
all the subsequent history of the hemispheres, since the lateral 
movement of the ventral area is reduced to a minimum. 
This results necessarily in the major part of the evagination 
being directed slightly upward but mostly forward toward the 
anterior pole of the embryo (figs. 17 and 18). The internal con- 
figuration during this process has undergone a marked change. 
The median telencephalic ventricle is still but a narrow slit, . 
while the forward growth of the hemisphere has developed the 
beginnings of the lateral telencephalic ventricle. The locus of 
the di-telencephalic groove is clearly seen through the presence of 
a cell-free area between the eminentia thalami and the dorsal 
part of the hemisphere (fig. 21). The ventral region of the evagi- 
nation shows a slight lateral growth and the production of an 
interhemispheric groove in the mid-ventral line. The caudal 
boundary of the evagination is still rather indistinct, since little 
outpouching has occurred at this point. But the shallow sulcus 
limitans may be seen coursing past the posterior limit of the 
velum transversum to be lost in the preoptic recess. No mark- 
ings on the medial wall of the hemisphere can be distinguished as 
yet, only the deep groove made by the anterior continuation of 
the sulcus interencephalicus anterior being evident. The internal 
structure of the hemisphere has undergone a marked change 
(fig. 21). The loosely packed arrangement of cells of the earlier 
stages has given place to a tightly organized neural wall. The 
rapid proliferation that has brought this about has resulted in a 
great increase in the number of cells, organizing the primitive 
arrangement into the ependymal, mantle and marginal layers. 
This elaboration is concomitant with the ingrowth of the olfac- 
tory nerve. While the presence of these fibers was undoubtedly 
the primitive stimulus producing the growth, the pattern of 
development has become so firmly fixed that the absence of ol- 
factory fibers does not preclude differentiation. A slightly looser 
arrangement of cells in the lateral region presages the formation 
of the lateral forebrain tract. This tract appears as a thin band 
of peripherally arranged fibers in the pars ventro-lateralis of the 
hemisphere. The tract becomes more robust as the neural tube 
is followed caudally, indicating that at this time the majority of 
