UNDEVELOPED CELLS.--Professor Donaldson tells us on this point that: 'At
birth, and for a long time after, many [nervous] systems contain cell
elements which are more or less immature, not forming a functional part
of the tissue, and yet under some conditions capable of further
development.... For the cells which are continually appearing in the
developing cortex no other source is known than the nuclei or granules
found there in its earliest stages. These elements are metamorphosed
neuroblasts--that is, elementary cells out of which the nervous matter
is developed--which have shrunken to a volume less than that which they
had at first, and which remain small until, in the subsequent process of
enlargement necessary for their full development, they expand into
well-marked cells. Elements intermediate between these granules and the
fully developed cells are always found, even in mature brains, and
therefore it is inferred that the latter are derived from the former.
The appearances there also lead to the conclusion that many elements
which might possibly develop in any given case are far beyond the number
that actually does so.... The possible number of cells latent and
functional in the central system is early fixed. At any age this number
is accordingly represented by the granules as well as by the cells which
have already undergone further development. During growth the proportion
of developed cells increases, and sometimes, owing to the failure to
recognize potential nerve cells in the granules, the impression is
carried away that this increase implies the formation of new elements.
As has been shown, such is not the case.'[1]
