Biology Reference
In-Depth Information
It should be noted that most of this information on metamorphic changes
of the tadpole eye is derived from analysis of
X. laevis
. Adult
X. laevis
remains
aquatic and relies more on smell than sight as compared to big-eyed terres-
trial frogs of fairy tales. It may be expected that these eye changes found in
X. laevis
are more extreme in terrestrial frogs, since they must see moving
prey in order to eat.
In
E. coqui
, there is early cell proliferation of the eye, so that the eye un-
dergoes three-fourth of its growth before the thyroid gland develops
(
Schlosser, 2008; Schlosser &Roth, 1997
). Unlike the ventral retinal growth
of metamorphosing
X. laevis
, cell division occurs throughout the
E. coqui
retina. Despite the rapid retinal growth, the major ipsilateral projections
do not occur until several weeks after hatching. This delay is odd, since
the froglets begin to eat live prey about a week after hatching. There is pres-
ently no information on the role of thyroid hormone in the development of
the
E. coqui
eye.
5.2. Limbs
The other frog structures that arise early in direct developers are the limbs. In
species with tadpoles, limb buds arise late, near the time that the tadpole be-
gins feeding, and they develop slowly. Hindlimb development accelerates
with increased thyroid hormone levels, and the start of metamorphic climax
is indicated by the emergence of the forelimb through a hole in the oper-
culum. When the primordium for the thyroid or pituitary gland was re-
moved or otherwise did not develop, there was a variable and limited
degree of hindlimb development, sometimes including a knee joint and
digits as well as some cartilage (
Allen, 1916, 1918, 1925; Hanaoka, 1967;
Hoskins & Hoskins, 1919; Kerney, Wassersug, & Hall, 2009; Kollros,
1961; Rot-Nikcevic & Wassersug, 2004; Terry, 1918
). On the other hand,
blockage of thyroid hormone synthesis with methimazole arrests limb devel-
opment in
X. laevis
early, without muscle or cartilage development (
Brown
et al., 2005
). Although there may be species differences, this methimazole
result raises the possibility that retention of a small amount of thyroid or pi-
tuitary tissue in the extirpation experiments allowed limb development to a
limited degree while not allowing metamorphosis.
In contrast, four large limb buds are present in
E. coqui
within a day after
closure of the neural folds (
Fig. 9.1
). The buds develop continuously over
the next 3 weeks to become the limbs of the free-living froglet (
Elinson,
1994; Townsend & Stewart, 1985
). There is a late requirement for
