The Questions of Developmental Biology

altering a target cell's sensitivity to growth factors or altering the amounts of growth factors
produced. Again, the vertebrate limb can provide a useful illustration. Local differences among
chondrocytes cause the central toe of the horse to grow at a rate 1.4 times that of the lateral toes
(Wolpert 1983). This means that as the horse grew larger during evolution, this regional
difference caused the five-toed horse to become a one-toed horse. A particularly dramatic
example of allometry in evolution comes from skull development. In the very young (4- to 5-mm)
whale embryo, the nose is in the usual mammalian position. However, the enormous growth of
the maxilla and premaxilla (upper jaw) pushes over the frontal bone and forces the nose to the top
of the skull (Figure 22.18). This new position of the nose (blowhole) allows the whale to have a
large and highly specialized jaw apparatus and to breathe while parallel to the water's surface
(Slijper 1962).
Fig. 22.18 Fig. 22.19
Allometry can also generate evolutionary novelty by small, incremental changes that
eventually cross some developmental threshold (sometimes called a bifurcation point). A change
in quantity eventually becomes a change in quality when such a threshold is crossed. It has been
postulated that this type of mechanism produced the external fur-lined "neck" pouches of pocket
gophers and kangaroo rats that live in deserts. External pouches differ from internal ones in that
they are fur-lined and they have no internal connection to the mouth. They allow these animals to
store seeds without running the risk of desiccation. Brylski and Hall (1988) have dissected the
heads of pocket gopher and kangaroo rat embryos and have looked at the way the external cheek
pouch is constructed. When data from these animals were compared with data from animals that
form internal cheek pouches (such as hamsters), the investigators found that the pouches are
formed in very similar manners. In both cases, the pouches are formed within the embryonic
cheeks by outpocketings of the cheek (buccal) epithelium into the facial mesenchyme (Figure
22.19). In animals with internal cheek pouches, these evaginations stay within the cheek.
However, in animals that form external pouches, the elongation of the snout draws up the
outpocketings into the region of the lip. As the lip epithelium rolls out of the oral cavity, so do the
outpockets. What had been internal becomes external. The fur lining is probably derived from the
external pouches' coming into contact with dermal mesenchyme, which can induce hair to form in
epithelia (see Chapter 12). Such a pouch has no internal opening to the mouth. Indeed, the
transition from internal to external pouch is one of threshold. The placement of the evaginations
anteriorly or posteriorly determines whether the pouch is internal or not. There is no "transitional
stage" having two openings, one internal and one external.* One could envision this
externalization occurring by a chance mutation or set of alleles that shifted the outpocketing to a
slightly more anterior location.