Development of New Organisms

Life Cycle of a Toad/ Frog

Toads are amphibians that reproduce during the rainy season. Males attract females by making loud croaking sounds. During mating, the male climbs onto the female’s back and externally fertilizes the eggs as the female lays them. The life cycle of a toad includes the following stages:

1. Egg Stage: The eggs are laid in jelly-like strings that protect them from harm.
2. Zygote Stage: The fertilized egg divides into an embryo, which feeds on the yolk and gets oxygen from the surrounding jelly.
3. Young Tadpole: After 1–2 days, the tadpole hatches and attaches itself to water plants using a sticky secretion. It feeds on yolk.
4. External Gill Stage: The tadpole develops external gills, jaws, and a long, coiled intestine. It begins to feed on plants.
5. Internal Gill Stage: Around 6–10 days after hatching, the external gills are replaced by internal gills, covered by an operculum. The tail grows longer, and muscles start to develop.
6. Limb Stage: Hind limbs grow first, followed by forelimbs. The lungs begin to develop, and the tadpole starts feeding on small animals. The tail shortens, and it starts transforming into a young toad.
7. Young Toad Stage: The tadpole loses its tail and fully becomes a toad, ready to live on land.

This process of transformation is called metamorphosis and is controlled by the hormone thyroxine. It takes about 40–45 days to complete.

Types of Metamorphosis

Metamorphosis is the transformation from a fertilized egg to an adult. Insects undergo two types of metamorphosis:

1. Complete Metamorphosis: Insects like butterflies and mosquitoes go through distinct stages: egg → larva → pupa → adult. The pupa looks very different from the adult insect.
2. Incomplete Metamorphosis: Insects like cockroaches develop from an egg into a nymph, which looks like a small adult but lacks wings and sexual maturity.

Life Cycle of a Housefly

Houseflies undergo complete metamorphosis, which includes the following stages:

1. Egg Stage: Female houseflies lay their eggs in moist environments. The eggs hatch into larvae within 8–24 hours.
2. Larva Stage: The maggot (larva) has a segmented body and uses hooks to move and feed. After 5–14 days, it transitions into the pupal stage.
3. Pupal Stage: The maggot forms a hard protective case called a puparium. After 3–10 days, the adult fly emerges.
4. Adult Stage: The adult fly, called imago, breaks out of the puparium and flies away once its wings dry.

Life Cycle of a Cockroach

1. Egg Stage: Female cockroaches lay 10–16 eggs in a protective case called an ootheca. These eggs hatch into nymphs after 30–100 days.
2. Nymph Stage: The nymphs grow, change color, and molt about 13 times before becoming adults.
3. Adult Stage: The cockroach gains wings during its final molts and matures into an adult in 11–20 months.

Eggs in Vertebrates

Most vertebrates, except mammals, lay eggs. The egg has several important parts:

• Shell: The hard, porous shell protects the egg and allows for gas exchange.
• Membranes: These protect the egg and the developing embryo.
• Albumen: Also called egg white, it provides nourishment for the embryo.
• Yolk: Rich in nutrients, the yolk feeds the embryo during development.
• Chalaza: A thick strand that helps keep the yolk in place.

Fish

Fish eggs are tiny and look like grains of sand. Each egg is enclosed in a case and provides nourishment for the embryo until it hatches.

Reptiles

Reptile eggs, such as those of the Agama lizard, have a tough, leathery shell that allows them to absorb water from the environment.

Birds

Bird eggs are the largest single cells. Their shells are porous to allow for gas exchange. The yolk nourishes the embryo, which is surrounded by membranes that protect it during development.

Mammals

Mammalian eggs are microscopic and develop inside the mother's body. The yolk provides initial nourishment before the placenta takes over.

Reproductive Strategies

Vertebrates have different strategies for reproduction:

• Oviparity: Eggs are laid in the environment, and the embryo develops outside the mother’s body.
• Viviparity: Embryos develop inside the mother's body, where they are nourished through the placenta, and are born alive.
• Ovoviviparity: Eggs are retained inside the mother's body without a placenta. The young hatch inside and are later born alive.

Embryo Development in Mammals

Mammalian embryos have several adaptations that help them survive:

1. Uterus: The embryo implants in the uterine wall for development.
2. Placenta: The placenta connects the embryo to the mother, allowing for nutrient and oxygen exchange.
3. Umbilical Cord: This cord links the embryo to the placenta, transporting blood and nutrients.
4. Embryonic Membranes:
   • Amnion: This membrane cushions the embryo with fluid and helps regulate its temperature.
   • Chorion: It aids in oxygen absorption.
   • Allantois: This membrane helps with respiration and waste disposal.