The Questions of Developmental Biology

6. The ectoderm gives rise to the epidermis, nervous system, and pigment cells.
7. The mesoderm generates the kidneys, gonads, bones, heart, and blood cells.
8. The endoderm forms the lining of the digestive tube and the respiratory system.
9. Karl von Baer's principles state that the general features of a large group of animals appear
earlier in the embryo than do the specialized features of a smaller group. As each embryo of a
given species develops, it diverges from the adult forms of other species. The early embryo of a
"higher" animal species is not like the adult of a "lower" animal.
10. Labeling cells with dyes shows that some cells differentiate where they form, while others
migrate from their original sites and differentiate in their new locations. Migratory cells include
neural crest cells and the precursors of germ cells and blood cells.
11. "Community of embryonic structure reveals community of descent" (Charles Darwin).
12. Homologous structures in different species are those organs whose similarity is due to their
sharing a common ancestral structure. Analogous structures are those organs whose similarity
comes from their serving a similar function (but which are not derived from a common ancestral
structure).
13. Congenital anomalies can be caused by genetic factors (mutations, aneuploidies,
translocations) or by environmental agents (certain chemicals, certain viruses, radiation).
14. Syndromes consists of sets of developmental abnormalities that "run together."
15. Organs that are linked in developmental syndromes share either a common origin or a
common mechanism of formation.
16. If growth is isometric, a twofold change in weight will cause a 1.26-fold expansion in length.
17. Allometric growth can create dramatic changes in the structure of organisms.
18. Complex patterns may be self-generated by reaction-diffusion events, wherein the activator of
a local phenomenon stimulates the production of more of itself as well as the production of a
more diffusible inhibitor.
2. Life cycles and the evolution of developmental patterns
Traditional ways of classifying catalog animals according to their adult structure. But, as
J. T. Bonner (1965) pointed out, this is a very artificial method, because what we consider an
individual is usually just a brief slice of its life cycle. When we consider a dog, for instance, we
usually picture an adult. But the dog is a "dog" from the moment of fertilization of a dog egg by a
dog sperm. It remains a dog even as a senescent dying hound. Therefore, the dog is actually the
entire life cycle of the animal, from fertilization through death.
The life cycle has to be adapted to its environment, which is composed of nonliving
objects as well as other life cycles. Take, for example, the life cycle of Clunio marinus, a small
fly that inhabits tidal waters along the coast of western Europe. Females of this species live only