Unit 7
Development Biology
Chapter 7.2
Development of FrogClass 12 Biology – Development of Frog Notes PDF
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Introduction
Frog is a useful vertebrate example for understanding early embryonic development because fertilization and much of early development occur externally in water.
1. Fertilization
Frog fertilization is generally external. Eggs and sperm are released into water, where a sperm fuses with an egg to form a diploid zygote.
- The egg has an animal pole and vegetal pole.
- The animal hemisphere contains less yolk and divides faster.
- The vegetal hemisphere contains more yolk and divides more slowly.
Fig. 1 – Frog Egg: Animal and Vegetal Poles
2. Cleavage
Frog cleavage is complete but unequal because yolk is concentrated toward the vegetal pole. Successive divisions produce 2-cell, 4-cell, 8-cell and multicellular stages.
Fig. 2 – Cleavage Stages in Frog
Fig. 3 – Morula
3. Blastulation
Continued cleavage produces a blastula. A fluid-filled cavity called the blastocoel develops inside.
Fig. 4 – Frog Blastula
4. Gastrulation
- Ectoderm forms the outer layer.
- Mesoderm forms the middle layer.
- Endoderm forms the inner layer.
During gastrulation, the primitive gut or archenteron develops and the blastopore becomes evident.
Fig. 5 – Simplified Frog Gastrula
5. Neurulation and Early Organ Formation
After gastrulation, the ectoderm along the dorsal side forms a neural plate. The neural plate folds and closes to form the neural tube, which later develops into the brain and spinal cord.
Fig. 6 – Neurulation
6. Tadpole Larva
The frog embryo develops into an aquatic tadpole with a tail, gills and larval feeding structures. As development proceeds, hind limbs and then forelimbs appear.
Fig. 7 – Tadpole
7. Metamorphosis
Metamorphosis is the transformation of the tadpole into the adult frog. Major changes include limb development, tail resorption, replacement of gills by lungs, changes in the digestive system and transition toward terrestrial life.
Fig. 8 – Frog Life Cycle
8. Quick Revision & Exam Points
- Describe fertilization in frog.
- What are animal and vegetal poles?
- Define cleavage and describe early cleavage stages.
- Draw a frog blastula.
- Define gastrulation and name the three germ layers.
- What is neurulation?
- Describe tadpole development.
- Define metamorphosis and list its major changes.
- Frog fertilization is usually external.
- Cleavage forms blastomeres.
- Morula = solid cell mass.
- Blastula contains blastocoel.
- Gastrulation produces ectoderm, mesoderm and endoderm.
- Neural tube forms from ectoderm.
- Tadpole is aquatic.
- Metamorphosis produces adult frog.
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