Glossary - The Origin of Individuals - page 217

individuals forming species change characteristics and evolve
through accumulating variations.
Neo-Darwinism: See Evolutionary synthesis .
Neural crest : Region (dorsal to the neural tube) in vertebrate
embryos. Its cells migrate in the embryo and give rise to several cell
lines including the cells of the nervous system.
Neuron circuits (networks) : See Neuron .
Neuron : Neurons are cells of the nervous system by means of which
nerve impulses are transmitted. In general they are elongated in
shape, with many branches (the dendrites) at one end and the
other end less branched (ending in synapses). Nerve impulses are
transmitted from one neuron to another through the connections
which are established between synapses and dendrites. The neu-
rons thus form circuits in which impulses circulate. Owing to the
very large number of dendrites, numerous different circuits can
form endowing the nervous system with great plasticity, due to
which it can respond to the numerous situations that may confront
the organism.
Neuronal cell : See Neuron .
Nominalism : In the debate on the species, which has never abated,
nominalism asserts that only individuals really exist. The nominal-
ist does not deny that certain individuals that are classified in the
same species resemble each other but this resemblance does not arise
from a constitutive principle, or essence, inherent in the individuals.
In contrast, for realists the species is perfectly real. It is a structure
shared by several individuals, and is the result of a constitutive prin-
ciple common to those individuals (see Essentialism ).
Nucleosome: The nucleosome is the basic unit in chromatin fibre.
It is a 'bead' of proteins, called histones, around which the DNA is