INTELLIGENT DEVELOPMENT 139Take just one small aspect of the early embryo, the neural crest, a tran-
sient pool of migratory cells unique to vertebrates. Under the micro-
scope, it can be seen to form from the folding of a sheet of cells into a kind
of tube— the neural tube. Some cells in this tube give rise to the spinal
cord. But others creep in diff er ent directions in the embryo to generate
a further diversity of cell types and tissues. Th ey include neurons of the
autonomic ner vous system (ultimately regulating body organs); dozens of
diff er ent nerve cells for the central ner vous system (brain); sensory
neurons of touch, smell, hearing, and vision; endocrine cells (producing
hormones in adrenal and thyroid glands); vari ous cardiac tissue cells;
pigment cells of the skin and internal organs; and the blood vessels. Neural
crest cells also give rise to the cranial tissues that generate facial bone and
cartilage, the cornea of the eye, meninges (membranes around the brain),
roots for teeth and eye muscles, and many others.
All the other tissues of the body emerge from the primordial, toti-
potent stem cells in similar ways. Th e body axis of vertebrates starts, for
example, as seria l repetitions of clumps of cells ca lled “segments” or “meta-
meres.” Th e residue of this pro cess is especially con spic u ous in the adult’s
spinal column. From these basic segments, the rest of the general body
form— bones, muscles, skin, limbs, and organs— emerges. Th is is creation
of a vast variety of cells, none of which could have been predicted from
even the fullest specifi cation of the cells’ DNA (fi gure 5.2).
Th e proliferation also involves not only changes of cell form but
massive migrations over long distances. It inevitably entails growth, or
just getting bigger, due to multiplication of cells through binary fi ssion.
But this growth and diff erentiation both happen in a perfectly coordi-
nated manner, preserving the required proportions of size, location, and
timing of development among diff er ent tissues.
It seems obvious to even the casual observer that such a highly inte-
grated and harmonious pro cess must be under the close supervision of
some executive function. But it is not. It all progresses through a remark-
able extension and amplifi cation of the intelligent systems already appar-
ent in single cells.
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