Figure 2.1 uses the development of a frog to show a representative life cycle. Let us look at this life cycle in a bit more detail. First, in most frogs, gametogenesis and fertilization are seasonal events for this animal, because its life depends upon the plants and insects in the pond where it lives and on the temperature of the air and water. A combination of photoperiod (hours of daylight) and temperature tells the pituitary gland of the female frog that it is spring. If the frog is mature, the pituitary gland secretes hormones that stimulate the ovary to make estrogen. Estrogen is a hormone that can instruct the liver to make and secrete the yolk proteins, which are then transported through the blood into the enlarging eggs in the ovary.^* The yolk is transported into the bottom portion of the egg (Figure 2.2A).

Figure 2.2

Early development of the frog Xenopus laevis. (A) As the egg matures, it accumulates yolk (here stained yellow and green) in the vegetal cytoplasm. (B) Frogs mate by amplexus, the male grasping the female around the belly and fertilizing the eggs as they (more...)

Another ovarian hormone, progesterone, signals the egg to resume its meiotic division. This is necessary because the egg had been “frozen” in the metaphase of its first meiosis. When it has completed this first meiotic division, the egg is released from the ovary and can be fertilized. In many species, the eggs are enclosed in a jelly coat that acts to enhance their size (so they won't be as easily eaten), to protect them against bacteria, and to attract and activate sperm.

Sperm also occur on a seasonal basis. The male leopard frogs make their sperm in the summer, and by the time they begin hibernation in autumn, they have all the sperm that are to be available 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.

Figure 2.3

Continued development of Xenopus laevis. (A) Gastrulation begins with an invagination, or slit, in the future dorsal side of the embryo. (B) This slit, the dorsal blastopore lip, as seen from the ventral surface (bottom) of the embryo. (C) The slit becomes (more...)

Organogenesis begins when the notochord—a rod of mesodermal cells in the most dorsal portion of the embryo—tells the ectodermal cells above it that they are not going to become skin. Rather, these dorsal ectoderm cells are to form a tube and become the nervous system. At this stage, the embryo is called a neurula. The neural precursor cells elongate, stretch, and fold into the embryo (Figure 2.3A-D), forming the neural tube. The future back epidermal cells cover them. The cells that had connected the neural tube to the epidermis become the neural crest cells. The neural crest cells are almost like a fourth germ layer. They give rise to the pigment cells of the body (the melanocytes), the peripheral neurons, and the cartilage of the face. Once the neural tube has formed, it induces changes in its neighbors, and organogenesis continues. The mesodermal tissue adjacent to the notochord becomes segmented into somites, the precursors of the frog's back muscles, spinal cord, and dermis (the inner portion of the skin). These somites appear as blocks of mesodermal tissue (Figure 2.3F,G). The embryo develops a mouth and an anus, and it elongates into the typical tadpole structure. The neurons make their connections to the muscles and to other neurons, the gills form, and the larva is ready to hatch from its egg jelly. The hatched tadpole will soon feed for itself once the yolk supply given it by its mother is exhausted (Figure 2.3H).

Metamorphosis of the tadpole larva into an adult frog is one of the most striking transformations in all of biology (Figure 2.4). In amphibians, metamorphosis is initiated by hormones from the tadpole's thyroid gland, and these changes prepare an aquatic organism for a terrestrial existence. (The mechanisms by which thyroid hormones accomplish these changes will be discussed in Chapter 18.) In anurans (frogs and toads), the metamorphic changes are most striking, and almost every organ is subject to modification. The changes in form are very obvious. For locomotion, the hindlimbs and forelimbs differentiate as the paddle tail recedes. The cartilaginous skull of the tadpole is replaced by the predominantly bony skull of the young frog. The horny teeth the tadpole uses to tear up pond plants disappear as the mouth and jaw take a new shape, and the fly-catching tongue muscle of the frog develops. Meanwhile the large intestine characteristic of herbivores shortens to suit the more carnivorous diet of the adult frog. The gills regress, and the lungs enlarge.

Figure 2.4

Metamorphosis of the frog Rana. (A) Huge changes are obvious when one contrasts the tadpole and the adult bullfrog. Note especially the differences in jaw structure and limbs. (B) Premetamorphic tadpole. (C) Prometamorphic tadpole, showing hindlimb growth. (more...)

As metamorphosis ends, the development of the first germ cells begins. In Rana pipiens, egg development lasts 3 years. At that time, the frog is sexually mature and can produce offspring of her own. The speed of metamorphosis is carefully keyed to environmental pressures. In temperate regions, for instance, metamorphosis must occur before the pond becomes frozen. A Rana pipiens frog can burrow into the mud and survive the winter; its tadpole cannot.

Since the bottom half of the egg usually contains the yolk, it divides more slowly (because the large yolk deposits interfere with cleavage). This portion is the vegetal hemisphere of the egg. Conversely, the upper half of the egg usually has less yolk and divides faster. This upper portion is called the animal hemisphere of the egg.^†