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Principles of Development
Chapter 8

Early Concepts: Preformation vs
 The

question of how a zygote
becomes an animal has been
asked for centuries.
 As recently as the 18th century,
the prevailing theory was a notion
called preformation – the idea
that the egg or sperm contains an

preformed miniature infant, or
“homunculus,” that simply
becomes larger during

Early Concepts: Preformation vs
 Kaspar

Friederich Wolff (1759)
demonstrated there was no preformed
chick in the early egg.
 Undifferentiated

granular material became
arranged into layers.
 The layers thickened, thinned, and folded to
produce the embryo.

Early Concepts: Preformation vs
 Epigenesis

is the concept that the
fertilized egg contains building materials
only, somehow assembled by an
unknown directing force.
 Although current ideas of development
are essentially epigenetic in concept, far
more is known about what directs growth
and differentiation.

Key Events in Development
 Development

describes the
changes in an
organism from its
earliest beginnings
through maturity.
 Search


Key Events in Development
 Specialization

of cell types occurs as a
hierarchy of developmental decisions.
Cell types arise from conditions created in
preceding stages.
 Interactions become increasingly restrictive.

 With

each new stage:

Each stage limits developmental fate.
 Cells lose option to become something different


Said to be determined.

Key Events in Development
 The

two basic processes responsible for
this progressive subdivision:
 Cytoplasmic
 Induction


 Fertilization

is the initial event in
development in sexual reproduction.
 Union

of male and female gametes
 Provides for recombination of paternal and
maternal genes.
 Restores

 Activates

the diploid number.

the egg to begin development.

 Oocyte
 Egg


grows in size by accumulating yolk.

 Contains

much mRNA, ribosomes, tRNA and
elements for protein synthesis.

 Morphogenetic

determinants direct the
activation and repression of specific genes
later in post-fertilization development.
 Egg nucleus grows in size, bloated with
 Now

called the germinal vesicle.

 Most

of these preparations in the egg occur
during the prolonged prophase I.
 In


 Oocyte

now has a highly structured system.

 After

fertilization it will support nutritional...
Principles of Development
Chapter 8
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