What stage would you find more specialized cells in a frogs life cycle?
Answers
e Frog Life Cycle
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.
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.
Answer:
It includes three stages: Egg, larva, and adult
The life cycle of a frog consists of three stages: egg, larva, and adult. As the froggrows, it moves through these stages in a process known as metamorphosis.