The Questions of Developmental Biology

mouth, whereas the frog tadpole produces mucus-secreting glands and suckers (Figure8).
The frog tadpole also has a horny jaw
without teeth, whereas the salamander
has a set of calcareous teeth in its jaw.
The larvae resulting from the
transplants were chimeras. The
salamander larvae had froglike
mouths, and the frog tadpoles had
salamander teeth and balancers. In
other words, the mesodermal cells
instructed the ectoderm to make a
mouth, but the ectoderm responded by
making the only kind of mouth it
"knew" how to make, no matter how
inappropriate.
Thus, the instructions sent by the
mesenchymal tissue can cross species
barriers. Salamanders respond to frog
signals, and chick tissue responds to
mammalian inducers. The response of
the epithelium, however, is speciesspecific.
So, whereas organ type
specificity (e.g., feather or claw) is
usually controlled by the mesenchyme
within a species, species specificity is usually controlled by the responding epithelium. As we
will see in Chapters 21 and 22, large evolutionary changes can be brought about by changing the
response to a particular inducer.
*Often, these inductions are called "secondary" inductions, whereas the tissue interactions that generate the neural tube
are called "primary embryonic induction." However, there is no difference in the molecular nature of "primary" and
"secondary" inductions. Primary embryonic induction will be detailed separately in Chapters 10 and 11.
When describing lens induction, one has to be careful to mention which species one is studying because there are
numerous species-specific differences. In some species, induction will not occur at certain temperatures. In other
species, the entire ectoderm can respond to the optic vesicle by forming lenses. These species-specific differences have
made this area very difficult to study (Jacobson and Sater 1988; Saha et al. 1989; Saha et al. 1991).
It is easy to distinguish permissive and instructive interactions by an analogy with a more familiar situation. This
textbook is made possible by both permissive and instructive interactions. The reviewers can convince me to change
the material in the chapters. This is an instructive interaction, as the information in the book is changed from what it
would have been. However, the information in the book could not be expressed without permissive interactions with
the publisher and printer.
Spemann is reported to have put it this way: "The ectoderm says to the inducer, 'you tell me to make a mouth; all
right, I'll do so, but I can't make your kind of mouth; I can make my own and I'll do that'" (quoted in Harrison 1933).