The Questions of Developmental Biology

for the following spring's breeding season. In most species of frogs, fertilization is external. The
male frog grabs the female's back and fertilizes the eggs as the female frog releases them (Figure
2.2B). Rana pipiens usually lays around 2500 eggs, while the bullfrog, Rana catesbiana, can lay
as many as 20,000. Some species lay their eggs in pond vegetation, and the jelly adheres to the
plants and anchors the eggs (Figure 2.2C). Other species float their eggs into the center of the
pond without any support.
Fertilization accomplishes several
things. First, it allows the egg to complete its
second meiotic division, which provides the egg
with a haploid pronucleus. The egg pronucleus
and the sperm pronucleus will meet in the egg
cytoplasm to form the diploid zygotic nucleus.
Second, fertilization causes the cytoplasm of the
egg to move such that different parts of the
cytoplasm find themselves in new locations
(Figure 2.2D). Third, fertilization activates those
molecules necessary to begin cell cleavage and
development (Rugh 1950). The sperm and egg
die quickly unless fertilization occurs.
During cleavage, the volume of the frog
egg stays the same, but it is divided into tens of
thousands of cells (Figure 2.2E-H). The animal
hemisphere of the egg divides faster than the
vegetal hemisphere does, and the cells of the
vegetal hemisphere become progressively larger
the more vegetal the cytoplasm. A fluid-filled
cavity, the blastocoel, forms in the animal
hemisphere (Figure 2.2H). This cavity will be important for allowing cell movements to occur
during gastrulation.
Gastrulation in the frog begins at a point on the embryo surface roughly 180 degrees
opposite the point of sperm entry with the formation of a dimple, called the blastopore. Cells
migrate through the blastopore and toward the animal pole (Figure 2.3A,B). These cells become
the dorsal mesoderm. The blastopore expands into a circle (Figure 2.3C), and cells migrating
through this circle become the lateral and ventral mesoderm. The cells remaining on the outside
become the ectoderm, and this outer layer expands vegetally to enclose the entire embryo. The
large yolky cells that remain at the vegetal hemisphere (until they are
encircled by the ectoderm) become the endoderm. Thus, at the end of
gastrulation, the ectoderm (the precursor of the epidermis and nerves)
is on the outside of the embryo, the endoderm (the precursor of the gut
lining) is on the inside of the embryo, and the mesoderm (the
precursor of connective tissue, blood, skeleton, gonads, and kidneys)
is between them.