The Questions of Developmental Biology

Embryology provides an endless assortment of fascinating animals and problems to study.
In this text, we will use but a small sample of them to illustrate the major principles of animal
development. This sample is an incredibly small collection. We are merely observing a small
tidepool within our reach, while the whole ocean of developmental phenomena lies before us.
After a brief outline of the experimental and genetic approaches to developmental biology,
we will investigate the early stages of animal embryogenesis: fertilization, cleavage, gastrulation,
and the establishment of the body axes. Later chapters will concentrate on the genetic and cellular
mechanisms by which animal bodies are constructed. Although an attempt has been made to
survey the important variations throughout the animal kingdom, a certain deuterostome
chauvinism may be apparent. (For a more comprehensive survey of the diversity of animal
development across the phyla, see Gilbert and Raunio 1997.)
*Plants undergo equally complex and fascinating patterns of embryonic and postembryonic development. However,
plant development differs significantly from that of animals, and the decision was made to focus this text on the
development of animals. Readers who wish to discover some of the differences are referred to Chapter 20, which
provides an overview of plant life cycles and the patterns of angiosperm (seed plant) development.
In mammals, the chorion is modified to form the placenta
to produce evolutionary change.
another example of the modification of development
Principles of Development: Life Cycles and Developmental Patterns
1. The life cycle can be considered a central unit in biology. The adult form need not be
paramount. In a sense, the life cycle is the organism.
2. The basic life cycle consists of fertilization, cleavage, gastrulation, germ layer formation,
organogenesis, metamorphosis, adulthood, and senescence.
3. Reproduction need not be sexual. Some organisms, such as Volvox and Dictyostelium, exhibit
both asexual reproduction and sexual reproduction.
4. Cleavage divides the zygote into numerous cells called blastomeres.
5. In animal development, gastrulation rearranges the blastomeres and forms the three germ
layers.
6. Organogenesis often involves interactions between germ layers to produce distinct organs.
7. Germ cells are the precursors of the gametes. Gametogenesis forms the sperm and the eggs.
8. There are three main ways to provide nutrition to the developing embryo: (1) supply the
embryo with yolk; (2) form a larval feeding stage between the embryo and the adult; or (3) create
a placenta between the mother and the embryo.
9. Life cycles must be adapted to the nonliving environment and interwoven with other life
cycles.