212
GEORGE SZÉKELY
of the synaptic connections were determined in the prefunctional stage of
nervous development.
c. On the basis of the cytoarchitectural structure of the nervous
system, anatomically well-defined regions can be distinguished from the
cord up to the cortex. The general shape and form of neurons, such as
the size of perikarya and the length and arborization pattern of processes,
are basically the same, except for a few highly specialized types of
neurons, in each region in all vertebrates including man. Data from the
literature indicate that this division of the nervous system begins at
early embryonic stages, and before the nervous system formation is completed the principal regions of the brain and cord are definitely determined. Cells emerging from the neural epithelium give definite signs of
specification. Progressive determination narrows the regulatory capacity
and interchangeability of the parts. Whereas bilateral extirpations of
the medullary plate are irremediable, unilateral ablations become reconstituted from the intact contralateral side (Harrison, 1947; Detwiler,
1947; Holtzer, 1951; Corner, 1963). Some easily identifiable cells (such
as Mauthner cells, Rohon-Beard cells, or motor cells) are missing or
appear in smaller numbers in the reconstituted half of the hindbrain and
cord (Stefanelli, 1951; DuShane, 1938; Ferguson, 1957; Stephens, 1959).
Regionally characteristic cells develop in tissue culture or from the
excised parts of the medullary tube in explantation experiments (Stefanelli, 1951; Corner, 1962). Extirpation experiments performed on the
medullary tube of salamander (Holtzer, 1951) and chicken (E. L. Wenger,
1950) embryos reveal a mosaic pattern of determination of the neural
epithelium, with each component being independent of the other, incapable of regeneration or regulation, and capable of producing only a
limited number and certain types of cells.
In advanced stages of neurohistogenesis, regionally characteristic
cell proliferations and migrations lead to the establishment of quantitative relations of the cell population in various sections of the central
nervous system. The extensive studies of Detwiler (1936) and Hamburger (1956) indicate that these processes are under the control of
periphery. A causal analysis of the phenomenon, however, has shown that
the periphery primarily affects the maintenance of cells rather than the
initiation of their proliferation (Hamburger and Levi-Montalcini, 1949),
and it has been emphasized that peripheral factors control merely the
quantitative aspects of a process that is already well under way when
the extrinsic control mechanism begins to operate. The observation that
a considerable increase in the number of motor cells in the spinal cord
cannot be achieved by overloading the periphery (Bueker, 1945) shows
that the number of certain cell types in some regions may be definitely
GEORGE SZÉKELY
of the synaptic connections were determined in the prefunctional stage of
nervous development.
c. On the basis of the cytoarchitectural structure of the nervous
system, anatomically well-defined regions can be distinguished from the
cord up to the cortex. The general shape and form of neurons, such as
the size of perikarya and the length and arborization pattern of processes,
are basically the same, except for a few highly specialized types of
neurons, in each region in all vertebrates including man. Data from the
literature indicate that this division of the nervous system begins at
early embryonic stages, and before the nervous system formation is completed the principal regions of the brain and cord are definitely determined. Cells emerging from the neural epithelium give definite signs of
specification. Progressive determination narrows the regulatory capacity
and interchangeability of the parts. Whereas bilateral extirpations of
the medullary plate are irremediable, unilateral ablations become reconstituted from the intact contralateral side (Harrison, 1947; Detwiler,
1947; Holtzer, 1951; Corner, 1963). Some easily identifiable cells (such
as Mauthner cells, Rohon-Beard cells, or motor cells) are missing or
appear in smaller numbers in the reconstituted half of the hindbrain and
cord (Stefanelli, 1951; DuShane, 1938; Ferguson, 1957; Stephens, 1959).
Regionally characteristic cells develop in tissue culture or from the
excised parts of the medullary tube in explantation experiments (Stefanelli, 1951; Corner, 1962). Extirpation experiments performed on the
medullary tube of salamander (Holtzer, 1951) and chicken (E. L. Wenger,
1950) embryos reveal a mosaic pattern of determination of the neural
epithelium, with each component being independent of the other, incapable of regeneration or regulation, and capable of producing only a
limited number and certain types of cells.
In advanced stages of neurohistogenesis, regionally characteristic
cell proliferations and migrations lead to the establishment of quantitative relations of the cell population in various sections of the central
nervous system. The extensive studies of Detwiler (1936) and Hamburger (1956) indicate that these processes are under the control of
periphery. A causal analysis of the phenomenon, however, has shown that
the periphery primarily affects the maintenance of cells rather than the
initiation of their proliferation (Hamburger and Levi-Montalcini, 1949),
and it has been emphasized that peripheral factors control merely the
quantitative aspects of a process that is already well under way when
the extrinsic control mechanism begins to operate. The observation that
a considerable increase in the number of motor cells in the spinal cord
cannot be achieved by overloading the periphery (Bueker, 1945) shows
that the number of certain cell types in some regions may be definitely