The Questions of Developmental Biology

cytoplasmic determinants can generate sea urchins that develop without larvae but still become
sea urchins. In fact, while all the vertebrates arrive at a particular stage of development called the
pharyngula, they do so by very different means (see Figure 1.5). Birds, reptiles, and fishes arrive
there after meroblastic cleavages of different sorts; amphibians get to the pharyngula stage by
way of radial holoblastic cleavage; and mammals reach the same stage after constructing a
blastocyst, chorion, and amnion. The earliest stages of development, then, appear to be extremely
plastic. Similarly, the later stages are very different, as the different phenotypes of mice, sunfish,
snakes, and newts amply demonstrate. There is something in the middle of development,
however, that appears to be invariant.
Raff (1994) argues that the formation of new body plans (Baupläne) is inhibited by the
need for global sequences of induction during the neurula stage (Figure 22.27). Before that stage,
there are few inductive events. After that stage, there are a great many inductive events, but
almost all of them occur within discrete modules. During early organogenesis, however, there are
several inductive events occurring simultaneously that are global in nature. At this stage, the
modules overlap and interact with one another. In vertebrates, to use von Baer's example, the
earliest stages of development involve specifying axes and undergoing gastrulation. Induction has
not yet happened on a large scale. Moreover, as Raff and colleagues have shown (Henry et al.
1989), there is a great deal of regulative ability at these stages, so small changes in morphogen
distributions or the position of cleavage planes can be accommodated. After the major body plan
is fixed, inductions occur all over the body, but are compartmentalized into discrete organforming
systems. The lens induces the cornea, but if it fails to do so, only the eye is affected.
Similarly, there are inductions in the skin that form feathers, scales, or fur. If they do not occur,
the skin or patch of skin may lack these structures, but the rest of the body is unchanged. But
during early organogenesis, the interactions are more global (Slack 1983). Failure to have the
heart in a certain place can affect the induction of eyes (see Figure 6.4). Failure to induce the
mesoderm in a certain region leads to malformations of the kidneys, limbs, and tail. It is this stage
that constrains evolution and that typifies the vertebrate phylum. Thus, once a vertebrate, it is
difficult to evolve into anything else.
Leibniz, probably the philosopher who most
influenced Darwin, noted that existence
must be limited not only to the possible but
to the compossible. That is, whereas
numerous things can come into existence,
only those that are mutually compatible will
actually exist (see Lovejoy 1964). So
although many developmental changes are
possible, only those that can integrate into
the rest of the organism (or which can cause
a compensatory change in the rest of the
organism) will be seen.
Canalization and the Release of Developmental Constraints
Not all mutations produce mutant phenotypes. Rather, development appears to be
buffered so that slight abnormalities of genotype or slight perturbations of the environment will
not lead to the formation of abnormal phenotypes (Waddington 1942). This phenomenon, called
canalization, serves as an additional constraint on the evolution of new phenotypes.