EARLY DEVELOPMENT OF HEMISPHERES - 285 
of the cells to the surrounding tissues? The probability is that 
neither of these factors operates to the exclusion of the other. 
For it has been shown that when the mass of nerve cells which is 
to form a cerebral hemisphere is transplanted to some other part 
of the larva, the general arrangement of the parts is maintained 
in the subsequent development (Burr, ’20). This would indicate 
that the inherent factor in the development of these ependyma 
cells was strong. But we have in the regenerating brain a new 
hemisphere formed from cells which, since they do not normally 
form a hemisphere, nevertheless acquire the inherent factors 
which determine the path or course of development. <A descrip- 
tion of the critical stages in this pathway of development follows. 
The medial aspect of the lateral wall of the neural tube of a 
stage 30 (Harrison) shows the rather strongly marked sulcus inter- 
encephalicus anterior curving from a point, just cephalic to the 
chiasmatic ridge caudally, to the lamina terminalis cephalically. 
The anterior third of this sulcus is more deeply marked than is 
the posterior two-thirds and represents the site of the future 
evagination of the hemisphere (fig. 14). In somewhat earlier 
stages the cephalic boundary of the neural tube sweeps in an 
almost unbroken curve from the mesencephalic region above to 
the hypothalmic region below. But in the stage-30 brain a 
sharp indentation is evident immediately above the cephalic 
termination of the -s. interencephalicus. It is produced ap- 
parently through the more rapid growth of the portions of the 
brain just above and below. By this process there is formed 
internally a crescentic ridge reaching horizontally from side to 
side of the neural tube, the velum transversum. Externally the 
indentation is carried caudally for a short distance to near the 
emerging optic tract, producing thereby the first signs of the di- 
telencephalic groove. It is bounded above by the prominent 
bulge of the thalamus. 
At this stage the wall of the neural tube presents no clearly 
defined division into mantle and marginal layers, the radially ar- 
ranged cells being scattered more or less uniformly through the 
wall (fig. 19). The wall of the tube in the region of the sulcus 
interencephalicus anterior shows a slight thickening and outward 
