Genes, Brains, and Human Potential The Science and Ideology of Intelligence

138 INTELLIGENT DEVELOPMENT

DEVELOPMENT AS IT APPEARS

From the start— the fusion of egg and sperm— development in the indi-
vidual is a pro cess of creating spectacular variation from seemingly
homogeneous beginnings, including identical genes. Th e single fertilized
egg is the zygote that, over the next few days, divides to form a ball of
two, four, eight, and then sixteen cells. Of course, this pro cess would
simply produce an ever- growing ball of undiff erentiated cells if it were
not regulated in some way. So the fi rst prob lem is how to start the pro-
cess of diff erentiation, or true development, into the beginnings of body
parts.
Over the fi rst few hours aft er conception, the start of diff erentiation
can be seen quite distinctly under a microscope. A cavity, the blastocoel,
forms and fi lls with fl uid to defi ne formation of the blastula and then the
gastrula (fi gure 5.1). Th e cells of the blastula have the amazing capacity to
become any tissue and cell type in the adult body. Th ey are the totipotent
stem cells. But they soon change to cells displaying varied sizes, shapes,
and types that come to form the gastrula. Now the cells are separated into
three layers: the ectoderm, mesoderm, and endoderm. Th ese form the
basis of the diff er ent cell types and tissues that follow with bewildering
variety.

gastrula

blastula

ectoderm

endoderm

archenteron

blastopore

blastocoel

FIGURE 5.1

Early cell division and formation of gastrula. (From Wikipedia.)

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