1126 THE STRUCTURE OF EVOLUTIONARY THEORY
of a homologous developmental pathway for the disparate eyes of two such different
phyla raised an obvious question about the generality of Pax- 6 as a "master control
gene" (using Gehring's phrase again) for all complex eyes, including the "paradigm
for convergent evolution" (Gehring, 1996, p. 14): the remarkable similarities in
function and structure between the single-lens eyes of vertebrates and cephalopods.
This case has persisted as a classic, ever since the formulation of convergence as
a concept, because the two eyes look so much alike, and work so similarly, despite
their separate origins from different tissues: the vertebrate eye as an evagination of
the brain, and the cephalopod eye by invagination of the epidermis. The squid eye
forms from a monolayer of epidermis that becomes thickened, multilayered and
internalized on the dorsal side of the head lobe. The outer ectodermal layer forms the
iris and the outer lens portion, while the inner half of the lens arises from the inner
ectodermal layer. Thus, the adult lens contains two parts, divided by a septum.
Meanwhile, the cornea, also of ectodermal origin, derives from a quite different
source on the edge of the arms, as they grow forward. In vertebrates, by contrast, the
optic vesicle arises as an evagination of the diencephalon, whereas the lens then
develops from overlying ectoderm. As the most interesting consequence of these
differences—well known, perhaps, because the vertebrate eye seems more "jury-
rigged" than the eye of the conventionally "inferior" squid on this basis—the polarity
of photoreceptors becomes inverted in vertebrates, but remains everted (an apparently
superior design) in cephalopods.
In a keenly anticipated result, Tomarev et al. (1997) found a homolog of
vertebrate and arthropod Pax- 6 in the squid Loligo opalescens. This gene is expressed
in the development of the embryonic eyes, olfactory organs, brain and arms. (This
common expression in visual and olfactory systems bears further study, especially
given the common ectodermal origin of both organs in vertebrates and their
embryonic interaction with adjacent regions of the