The Questions of Developmental Biology

There are several strands that weave together to form the anatomical approaches to
development. The first strand is comparative embryology, the study of how anatomy changes
during the development of different organisms. For instance, a comparative embryologist may
study which tissues form the nervous system in the fly or in the frog. The second strand, based on
the first, is evolutionary embryology, the study of how changes in development may cause
evolutionary changes and of how an organism's ancestry may constrain the types of changes that
are possible. The third anatomical approach to developmental biology is teratology, the study of
birth defects. These anatomical abnormalities may be caused by mutant genes or by substances in
the environment that interfere with development. The study of abnormalities is often used to
discover how normal development occurs. The fourth anatomical approach is mathematical
modeling, which seeks to describe developmental phenomena in terms of equations. Certain
patterns of growth and differentiation can be explained by interactions whose results are
mathematically predictable. The revolution in graphics technology has enabled scientists to model
certain types of development on the computer and to identify mathematical principles upon which
those developmental processes are based.
Evolutionary Embryology
Charles Darwin's theory of evolution restructured comparative embryology and gave it a
new focus. After reading Johannes Müller's summary of von Baer's laws in 1842, Darwin saw
that embryonic resemblances would be a very strong argument in favor of the genetic
connectedness of different animal groups. "Community of embryonic structure reveals
community of descent," he would conclude in On the Origin of Species in 1859.
Larval forms had been used for taxonomic classification even before Darwin. J. V.
Thompson, for instance, had demonstrated that larval barnacles were almost identical to larval
crabs, and he therefore counted barnacles as arthropods, not molluscs (Figure 1.12; Winsor 1969).
Darwin, an expert on barnacle taxonomy, celebrated this finding: "Even the illustrious Cuvier did
not perceive that a barnacle is a crustacean, but a glance at the larva shows this in an
unmistakable manner." Darwin's evolutionary interpretation of von Baer's laws established a
paradigm that was to be followed for many decades, namely, that relationships between groups
can be discovered by finding common embryonic or larval forms. Kowalevsky (1871) would
soon make a similar type of discovery (publicized in Darwin's Descent of Man) that tunicate
larvae have notochords and form their neural tubes and other organs in a manner very similar to
that of the primitive chordate Amphioxus. The tunicates, another enigma of classification schemes
(formerly placed, along with barnacles, among the molluscs), thereby found a home with the
chordates.
Darwin also noted that embryonic organisms sometimes make structures that are
inappropriate for their adult form but that show their relatedness to other animals. He pointed out
the existence of eyes in embryonic moles, pelvic rudiments in embryonic snakes, and teeth in
embryonic baleen whales.
Darwin also argued that adaptations that depart from the "type" and allow an organism to
survive in its particular environment develop late in the embryo.* He noted that the differences
between species within genera become greater as development persists, as predicted by von
Baer's laws. Thus, Darwin recognized two ways of looking at "descent with modification." One
could emphasize the common descent by pointing out embryonic similarities between two or
more groups of animals, or one could emphasize the modifications by showing how development
was altered to produce structures that enabled animals to adapt to particular conditions.