78 public questions tagged with this topic.
Hindlimbs: Muscular, elongated, and webbed with five digits. They propel the frog forward during leaping on land and paddling in water. Forelimbs: Short and unwebbed with four digits, designed to support the body and absorb landing impact.
Ref: NCERT Biology Textbook for Class XI and XII (Zoology section), Chapter: Structural Organisation in Animals and Frog, Topic: Tissues, skeletal and organ systems.
Male frogs have vocal sacs, which help in producing calls to attract females. This follows from latest NCERT 2026-27 principle explaining the concept clearly for NEET students in simple steps as per rationalized syllabus.
Ref: NCERT Biology Textbook - Latest Edition for Academic Session 2026-27 (Zoology section, Rationalized Textbook for Class XI and XII), Chapter: Biology - Zoology portion (Latest NCERT Textbooks for Academic Session 2026-27 - Rationalized Edition for Class XI and XII), Topic: Structural organization, physiology, human health and related concepts as per latest syllabus.
The nictitating membrane protects the frog’s eyes underwater and keeps them moist. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Biology Textbook for Class XI and XII (Zoology section), Chapter: Structural Organisation in Animals and Frog, Topic: Tissues, skeletal and organ systems.
Amphibians require moist environments because they undergo external fertilization and have aquatic larval stages. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Biology Textbook for Class XI and XII (Zoology section), Chapter: Principles of Inheritance, Molecular Basis of Inheritance and Biotechnology, Topic: Genetics, DNA techniques and applications.
In newt forelimb regeneration retinoic acid typically proximalizes positional identity reprogramming distal blastema toward proximal fate. Applied topically or via implanted beads, RA activates RAR-dependent transcription of Meis1/2 and proximal Hox9 genes, increases Prod1 expression level characteristic of proximal cells, suppresses distal HoxA13. Resulting regenerate duplicates proximal structures, converting wrist blastema into complete limb. Endogenous gradient with high RA proximally shapes normal pattern. Dose-dependent transformations demonstrated by Maden confirm proximalizing action; thus RA functions as respecifying morphogen resetting positional memory rather than eliminating identity or having no effect.
Ref: NCBI Bookshelf, Limb Regeneration: RA proximalization, Meis, Prod1 regulation in newt limb regeneration.
Classic induction discovered by Spemann and Mangold involves dorsal involuting mesoderm signaling to overlying ectoderm to change its fate. Chordamesoderm secreting soluble antagonists Chordin, Noggin, and Follistatin antagonizes BMP signaling in dorsal ectoderm, preventing epidermal fate and allowing neural differentiation forming neural plate. Ventral ectoderm under active BMP remains epidermis. Ectoderm-mesoderm interaction is therefore required for neural plate formation, illustrating embryonic induction where one germ layer influences fate of another through secreted factors rather than autonomous differentiation program, demonstrating paracrine regulation of cell fate during vertebrate organogenesis and patterning.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Ectoderm-mesoderm interaction in embryonic induction neural plate.
Spemann-Mangold organizer responsible for secondary axis induction derives from dorsal marginal zone. After cortical rotation, dorsal vegetal cells with nuclear beta-catenin and Nodal signaling induce overlying dorsal marginal zone to become organizer expressing goosecoid, chordin, noggin. At gastrulation, this region forms dorsal lip of blastopore containing bottle cells and involuting chordamesoderm forming notochord. Fate maps show dorsal lip contributes to prechordal plate and notochord, while ventral lip forms ventral mesoderm. Transplantation of dorsal lip to ventral side induces secondary axis with neural tissue, whereas ventral lip lacks organizing ability, confirming dorsal origin and inductive power.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Spemann organizer derived from dorsal lip of blastopore.
The blastocoel is a fluid-filled cavity formed during cleavage stages by active sodium pumping and tight junction sealing between superficial blastomeres. Positioned between animal cap and vegetal mass, it provides empty space required for involution and migration of mesodermal cells during gastrulation. Without blastocoel, involuting marginal zone cells would have no cavity to move into, preventing archenteron formation. Its roof thins via epiboly while floor consists of large yolky cells. Collapse and replacement by expanding archenteron marks progression of gastrulation and germ layer rearrangement.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Blastocoel function facilitates gastrulation movements.
Amphibian gastrulation initiates specifically where dorsal determinants have accumulated before cleavage. Following cortical rotation, gray crescent region opposite sperm entry acquires high nuclear beta-catenin, expresses organizer genes siamois and goosecoid, and forms bottle cells by apical constriction. This site becomes dorsal lip of blastopore, first point of marginal zone involution and archenteron initiation. Involution spreads laterally and ventrally around blastopore forming lateral and ventral lips. Initiation at gray crescent ensures chordamesoderm internalizes dorsally to form notochord beneath ectoderm where it secretes BMP antagonists inducing neural plate formation.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Amphibian gastrulation begins at gray crescent.
Gray crescent cytoplasm contains determinants accumulated via cortical rotation: dishevelled protein, GBP, kinesins and Wnt11 RNA enriched dorsally inhibiting GSK-3 beta. This permits beta-catenin nuclear accumulation in dorsal marginal cells overlaying crescent. These cells become Spemann organizer in dorsal blastopore lip expressing Goosecoid, Chordin and Noggin, inducing neural plate. Transplantation shows dorsal equatorial region derived from gray crescent involutes as chordamesoderm. Ventral blastopore cells lacking nuclear beta-catenin form blood and mesenchyme, not organizer activity, confirming developmental origin from gray crescent.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Spemann organizer derived from gray crescent equatorial region.
Upon fertilization, sperm entry triggers calcium wave and formation of microtubule array from sperm aster at vegetal cortex. Dynein-dependent cortical rotation shifts vegetal cortex 30 degrees relative to inner cytoplasm, transporting dorsal determinants like Dishevelled, GBP and Wnt11 mRNA toward opposite side of sperm entry. This opposite side shows depigmentation as melanosomes move, forming gray crescent rich in beta-catenin stabilizing factors. Future dorsal organizer arises there. Sperm entry thus defines ventral pole, gray crescent dorsal pole marking organizer position for gastrulation patterning.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Gray crescent formation opposite point of sperm entry via cortical rotation.
Initial breaking of radial symmetry in amphibians occurs at fertilization when sperm aster forms in vegetal cortex. Astral microtubules guide cortical rotation moving dorsal determinants 30 degrees away from sperm entry point. Sperm entry site therefore becomes ventral pole while opposite side accumulates Wnt11, Dishevelled and GBP, stabilizing beta-catenin dorsally. Gravity can bias but not determine axis; genetic differences and uterine orientation irrelevant in oviparous amphibians. Thus point of sperm entry sets dorso-ventral axis by directing cortical rotation direction and dorsal determinant transport. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.
Ref: Wolpert, Principles of Development, Chapter 4: Sperm entry and amphibian axis specification.
