Stages in the development of frog – Pre-embryonic, Embryonic and Post-embryonic Development

The development of frogs follows a typical sequence of events observed in sexually reproducing organisms. It begins with the fertilization of an egg or zygote. The zygote then undergoes a series of divisions, dividing and re-dividing to form an embryo. The embryo is the earliest stage of development and remains within the egg or reproductive organs of the mother until hatching or birth.

The study of embryo development is known as embryology, which is a branch of biology dedicated to understanding the processes and stages of embryonic growth. In almost all sexually reproducing organisms, including frogs, the development of the embryo follows a similar sequence of events.

In the case of frogs, the sexes are separate, with females being larger than males. Male frogs possess a nuptial pad at the base of the first finger of their forelimbs and also have a pair of vocal sacs. During mating, known as amplexus, the male grasps the female’s trunk with his forelimbs. Amplexus is a special kind of embrace where the male holds onto the female during reproduction. It is important to note that frogs and toads do not possess penises.

During amplexus, the female frog releases her eggs, typically into water, while the male sheds his sperm over the eggs. This allows for fertilization to occur externally, as the eggs are exposed to the surrounding water where the sperm can reach and fertilize them. This external fertilization is a common reproductive strategy observed in many amphibians, including frogs.

Sperm

• The sperm is a specialized reproductive cell that plays a crucial role in fertilization. In the context of this content, the mature sperm is described as having specific structural features. On average, the mature sperm measures approximately 0.03mm in length.
• The head of the sperm is elongated and solid, containing genetic material necessary for fertilization. Located at the anterior end of the head is a bead-like structure called the acrosome. The acrosome contains enzymes that aid in the penetration of the egg’s protective layers during fertilization.
• The middle piece of the sperm is described as short and is not visible in the provided information. The middle piece typically contains energy-producing mitochondria, which provide the necessary ATP (adenosine triphosphate) for sperm motility.
• The tail of the sperm, also known as the flagellum, is described as a gray filamentous extension. It is significantly longer than the sperm head, typically measuring four or more times its length. The tail is responsible for the sperm’s motility, allowing it to swim through the female reproductive tract in search of the egg for fertilization.
• Overall, the structure of the sperm is highly specialized for its function in fertilization. The head contains genetic material, the acrosome aids in egg penetration, and the tail provides the necessary mobility for reaching the egg.

Egg

The egg of a frog is approximately 2mm in diameter when it is ovulated. It is surrounded by two accessory egg membranes in addition to the plasma membrane. The first membrane, called the vitelline membrane, is a transparent non-living membrane formed by the ovum itself and is located just outside the plasma membrane. The second membrane is the jelly coat or albumen, which is secreted by the walls of the oviduct. When the egg comes into contact with water, the jelly coat absorbs water and swells up, providing protection against injury and infection by bacteria and other microorganisms.

The frog’s egg exhibits well-developed polarity and radial symmetry. The cytoplasm of the egg can be divided into two regions: the cortex and the endoplasm.

1. Egg Cortex: The cortex is a jelly-like viscous layer of cytoplasm that adheres to the plasma membrane. It contains membrane-bound spherical bodies called cortical granules, which contain acid mucopolysaccharides. These granules are arranged close to the plasma membrane. The animal hemisphere of the egg cortex contains dark-brown pigment granules, giving it a dark brown color, while the vegetal pole appears whitish with fewer pigment granules. The cortical layer is stable and does not shift with cytoplasmic movement or centrifugal force. It plays a crucial role in establishing the polarity, bilateral symmetry, and general organization of the developing egg.
2. Endoplasm: The inner ooplasm, along with its nucleus, is referred to as the endoplasm. It is colloidal in nature and contains cell organelles such as mitochondria and ribosomes, as well as organic and inorganic substances. The endoplasm also contains a cup-shaped mass of white yolk platelets known as the vitelline cupola. The germinal vesicle or nucleus is located near the animal pole of the egg. The distribution of yolk granules is uneven, with smaller and fewer yolk granules in the animal pole and a higher concentration of heavily deposited yolk in the vegetal pole. The frog’s egg is considered mesolecithal and moderately telolecithal, as it contains a moderate amount of yolk that is unevenly distributed in the cytoplasm, with the vegetal pole having the highest concentration.

In summary, the frog’s egg has distinct structural features including the vitelline membrane, jelly coat, cortex, and endoplasm. These features contribute to the polarity, symmetry, and overall organization of the developing egg during frog development.

Stages before and during embryonic development in living organisms

The stages before and during embryonic development in living organisms involve a series of processes that lead to the formation of a fully developed organism. Here is an overview of these stages:

1. Gametogenesis: Gametogenesis is the process of forming specialized cells called gametes or sex cells. In males, it is called spermatogenesis, which produces sperm cells, while in females, it is called oogenesis, which produces egg cells.
2. Mating: Mating involves the transfer of male gametes (sperm) into the female’s body through copulation or sexual intercourse. This is necessary for fertilization to occur.
3. Fertilization: Fertilization is the fusion of a male gamete (sperm) and a female gamete (egg) to form a zygote. It typically occurs in the female reproductive tract and marks the beginning of embryonic development.
4. Cleavage: Cleavage refers to the rapid and repeated cell divisions that take place in the zygote. These divisions result in the formation of a cluster of smaller cells called blastomeres. Cleavage partitions the cytoplasm of the zygote into many smaller cells.
5. Morulation: During morulation, the blastomeres undergo further divisions and rearrangements, resulting in the formation of a solid ball of cells called a morula. The morula is the early stage of embryo development.
6. Blastulation: Blastulation follows morulation and involves the formation of a hollow ball of cells called a blastula or blastocyst. The blastula consists of an outer layer of cells called the trophoblast and an inner cell mass.
7. Gastrulation: Gastrulation is a critical process in which cells of the blastula undergo movement and rearrangement to form three germ layers: ectoderm, endoderm, and mesoderm. These layers give rise to different tissues and organs in the developing organism.
8. Organogenesis: Organogenesis is the stage of development where the three germ layers give rise to specific organs and tissues. During this stage, the cells differentiate and organize themselves to form the structures and systems of the organism.
9. Morphogenesis: Morphogenesis refers to the growth and differentiation of form and structure in the developing organism. It involves the precise coordination of cell division, migration, and specialization to shape the body and establish its overall structure.
10. Growth: Growth is an ongoing process throughout embryonic development and beyond. It involves an increase in size and weight of the organism through both cell division and cell enlargement.

These stages collectively contribute to the complex process of embryonic development, leading to the formation of a fully developed and functional organism.

1. Gametogenesis

• Gametogenesis is the process by which specialized reproductive cells called gametes are produced in males and females. It involves the formation of sperm cells in males through spermatogenesis and the production of egg cells (ova) in females through oogenesis.
• In males, spermatogenesis takes place in the testes. It begins with the division of cells called spermatogonia through mitosis. These spermatogonia then undergo further divisions to form primary spermatocytes. The primary spermatocytes undergo meiosis I to produce two secondary spermatocytes. Meiosis II follows, resulting in the formation of four haploid spermatids. These spermatids further differentiate and undergo structural changes to become mature sperm cells, also known as spermatozoa.
• Sperm cells are microscopic and have a characteristic structure. They are thread-like in shape and measure about 0.03mm in length. Sperms have an elongated solid head that contains genetic material, including the nucleus. The head also has an anterior bead-like structure called the acrosome, which contains enzymes essential for fertilization. The middle piece of the sperm is short and invisible, while the tail, known as the flagellum, is a long, filamentous extension that provides motility to the sperm.
• In females, oogenesis occurs in the ovaries. It begins before birth when oogonia undergo mitotic divisions to form primary oocytes. These primary oocytes enter a dormant stage called prophase I of meiosis and remain arrested until puberty. At puberty, with each menstrual cycle, one primary oocyte resumes meiosis and progresses to meiosis I, producing one large secondary oocyte and a smaller polar body. The secondary oocyte then enters meiosis II but remains arrested in metaphase II unless fertilization occurs. If fertilized, meiosis II is completed, and a mature ovum or egg cell is formed.
• Ova are larger than sperm cells and are nearly spherical in shape. They are non-motile and contain a significant amount of cytoplasm to support the developing embryo. The eggs of frogs, like many other organisms, are mesolecithal and telolecithal. This means they contain a moderate amount of yolk that is distributed unevenly in the cytoplasm. The vegetal pole of the egg has the highest concentration of yolk, while the animal pole has fewer yolk granules.
• Overall, gametogenesis is a complex process that ensures the production of functional and specialized gametes, sperm in males and eggs in females, which are essential for sexual reproduction and the continuation of the species.

2. Mating

• Mating is the process in which male and female individuals come together for the purpose of reproduction. In the case of frogs, mating typically occurs during the breeding season, often in the rainy season. During this time, the male frogs produce characteristic calls or croaks to attract females.
• In frogs, mating involves a specific form of sexual embrace known as amplexus or pseudo-copulation. The male frog positions himself on the back of the female frog and clasps her body using his forelimbs. This embrace helps the male maintain physical contact with the female during the reproductive process.
• When the female frog is ready to release her eggs, she does so through a common opening called the cloaca. The cloaca serves as the exit for both waste and reproductive materials. The female sheds several hundred eggs into the water through her cloaca.
• As the female releases her eggs, the male simultaneously releases his spermatic fluid. The male deposits this fluid over the eggs in the surrounding water, which allows for the fertilization of the eggs. The sperm cells swim toward the eggs, and upon reaching them, the process of fertilization occurs. Fertilization is the fusion of the sperm and egg, resulting in the formation of a zygote, which develops into an embryo.
• Mating in frogs is a crucial step in their reproductive cycle, ensuring the successful fertilization of the eggs and the continuation of their species. The specific behaviors and mechanisms involved in mating vary among different species of frogs but generally involve the male and female coming together to facilitate the transfer of sperm to the eggs.

3. Fertilization

• Fertilization is the process by which the male and female gametes, the sperm and the ovum respectively, combine to form a new individual. In the case of frogs, fertilization is an external process that occurs in water.
• During fertilization in frogs, one sperm cell penetrates one ovum. As the male pronucleus enters the ovum, a second polar body is formed through the budding off of cellular material. The first polar body, which was already present below the vitelline membrane, is also formed during the maturation process of the ovum.
• Following the formation of the second polar body, the fusion of the male and female nuclei takes place. This fusion is known as fertilization. The resulting cell is called a zygote, and its nucleus is referred to as the zygote nucleus. The zygote nucleus contains a diploid chromosome number, which in the case of frogs is typically 26 chromosomes.
• After fertilization, the zygote begins to undergo a process called cleavage or segmentation. Cleavage involves rapid cell divisions of the zygote without significant growth. These divisions result in the formation of smaller cells known as blastomeres. The process of cleavage is essential for the subsequent development of the embryo.
• Fertilization marks the beginning of the development of a new individual. The zygote, formed through the fusion of genetic material from the sperm and ovum, contains the necessary genetic instructions for the development and growth of the organism.

4. Cleavage or segmentation

• Cleavage, also known as segmentation, is a process of rapid cell divisions that occur in the zygote following fertilization. In frogs, cleavage is considered holoblastic but unequal, meaning that the division is complete but the resulting cells are unequal in size.
• The first division of cleavage in frogs is vertical, dividing the zygote into two blastomeres. This division occurs along a furrow that extends from the animal pole to the vegetal pole, which is the lower end of the embryo.
• The second division is also vertical but occurs at a right angle to the first division, resulting in the formation of four blastomeres.
• The third division is horizontal, passing above the equator of the embryo, and gives rise to eight unequal blastomeres. Among these blastomeres, the four smaller upper ones are called micromeres, while the four larger lower ones are referred to as megameres.
• Following the third division, two more vertical divisions, known as the fourth cleavage set, take place, resulting in a total of 16 cells. Among these cells, there are eight megameres and eight micromeres.
• Subsequently, two horizontal divisions occur, leading to the formation of a 32-celled stage. This stage consists of cells arranged in a spherical arrangement.
• The process of cleavage is vital for the initial development of the embryo, as it establishes the basic cellular framework and begins to lay the groundwork for the subsequent stages of embryonic development. The unequal distribution of cells during cleavage sets the stage for further differentiation and specialization as the embryo progresses.

5. Morulation (formation of morula)

• Following the 32-cell stage in frog embryonic development, the subsequent cleavages become less regular and harder to track. There is a notable difference in the rate of division between micromeres and megameres. Micromeres, which contain less yolk, divide more rapidly compared to megameres, which have a higher concentration of yolk towards the vegetal end of the embryo.
• As a result of these irregular cell divisions, the zygote takes on a distinct appearance resembling a mulberry-shaped solid ball of cells. This stage is called the morula. The term “morula” derives from the Latin word for mulberry, reflecting the morula’s resemblance to the shape and texture of a mulberry fruit.
• The morula consists of a cluster of cells tightly packed together, forming a compact and solid structure. The cells of the morula are undifferentiated and relatively small in size. At this stage, the morula lacks a central cavity, which differentiates it from the subsequent stage known as the blastula.
• The formation of the morula marks an important milestone in embryonic development. It signifies the transition from the earlier stages of rapid cell divisions (cleavage) to a more complex arrangement of cells, setting the stage for further developmental processes. From the morula, the embryo proceeds to the next stage, known as blastulation, where a central fluid-filled cavity, called the blastocoel, forms within the cluster of cells.

6. Blastulation (formation of blastula)

Continuing from the morula stage, the division of blastomeres in frog embryonic development leads to a significant change in the arrangement of cells. The blastomeres arrange themselves at the periphery, creating a small central cavity filled with fluid. This fluid-filled cavity is called the blastocoel or segmentation cavity.

As a result, the embryo takes on the appearance of a hollow ball, and this stage is referred to as the blastula or coeloblastula. The process of the blastula’s formation is known as blastulation.

Although the blastula may appear to be composed solely of micromeres and megameres, specialized staining techniques can identify specific regions that will contribute to different parts of the future body. These regions can be categorized as follows:

1. Animal pole: This region represents the presumptive ectoderm, which is the outermost germ layer. The animal pole can be further divided into the presumptive epidermis (outer skin layer) and the neural plate, which will give rise to the nervous system.
2. Vegetal pole: Near the vegetal pole, a small area represents the presumptive notochord. The notochord is a flexible rod-like structure that serves as the precursor to the backbone.
3. Adjacent to the notochord: In close proximity to the notochord lies the presumptive mesoderm, which is the middle germ layer. The mesoderm gives rise to various tissues, including muscles, bones, and connective tissues.
4. Remaining vegetal pole: The remaining portion of the vegetal pole, consisting of large yolk-laden megameres, forms the future endoderm. The endoderm is the innermost germ layer and develops into organs such as the digestive tract, respiratory system, and associated glands.

Through blastulation, the blastula stage sets the foundation for subsequent developmental processes, including gastrulation, during which further cell movements and differentiations occur to shape the embryo’s body plan and initiate the formation of distinct tissue layers and organ systems.

7. Gastrulation (formation of gastrula)

After the blastula stage, the next important phase in embryonic development is gastrulation. Gastrulation involves a series of processes that transform the blastula into a gastrula, characterized by the formation of distinct germ layers. Gastrulation in frogs can be described in the following stages:

• a. Epiboly (overgrowth of micromeres): The micromeres, located at the animal pole, divide rapidly and expand over the larger megameres. This process encloses the presumptive notochord, mesoderm, and endoderm, while leaving a small area known as the yolk plug.
• b. Invagination: An invagination, or inward folding, occurs behind the presumptive notochord. This invagination marks the beginning of the formation of the archenteron, a primitive gut cavity. The opening of this invagination is referred to as the blastopore, with its anterior end identified as the dorsal lip of the blastopore. The archenteron cavity develops above the megameres.
• c. Involution or Migration of micromeres: The micromeres migrate inward from the dorsal lip of the blastopore, causing the archenteron to enlarge. The migrating micromeres form a thick layer on the dorsal surface of the archenteron, which will give rise to the notochord and mesoderm. As the archenteron develops, the blastopore decreases in size and eventually disappears. Micromeres also migrate on the sides and ventral surface of the dorsal lip, forming lateral and ventral lips respectively. These lips eventually unite, further reducing the size of the blastopore. The yolk-filled megameres exit through the blastopore, forming the yolk plug.
• d. Rotation of gastrula: During this stage, the gastrula rotates inside the vitelline membrane. The blastopore moves closer to the original vegetal pole of the embryo. The yolk plug moves inward and is positioned on the ventral surface of the archenteron. As gastrulation progresses, the blastocoel, the fluid-filled cavity of the blastula, disappears completely.

At the end of gastrulation, the gastrula exhibits three distinct layers:

1. Ectoderm: The outer surface of the gastrula, which will give rise to the future neural plate and epidermis.
2. Chordamesoderm: The cells on the roof of the archenteron, which will contribute to the formation of the notochord.
3. Endoderm: The floor and sides of the archenteron, which will develop into the endoderm, forming the primitive gut.

Although the chordamesoderm and endoderm are not clearly distinguishable at this stage, neurulation follows gastrulation, during which the notochord is formed from the chordamesoderm.

8. Organogenesis

After gastrulation, the next phase in embryonic development is organogenesis. Organogenesis involves the formation and differentiation of various embryonic tissues from the three primary germinal layers: ectoderm, mesoderm, and endoderm. This process ultimately leads to the development of distinct organs and structures in the embryo. In frogs, organogenesis progresses as follows:

a. Neurulation (formation of neural tube): On the mid-dorsal region of the embryo, the ectoderm cells thicken to form the neural plate, flanked by neural folds on either side. The neural folds gradually enlarge and fuse at the mid-dorsal region, forming the neural tube. The neural tube opens at the anterior end through a small opening called the anterior neuropore. The posterior end of the neural tube remains connected to the archenteron by a canal called the neurenteric canal. At this stage, the embryo is referred to as a neurula. Eventually, the neural tube differentiates into the brain at the anterior portion and the spinal cord at the posterior portion.

b. Formation of notochord and mesoderm: The chordamesoderm, located in the mid-dorsal region, forms a cylindrical rod-like structure known as the notochord. The remaining chordamesoderm gives rise to the mesoderm. The mesoderm can be further divided into three regions:

• Epimere: The dorsally situated part of the mesoderm, which further differentiates into three components:
  • Myotomes: These give rise to the body musculature.
  • Dermatomes: They form the dermis of the skin.
  • Sclerotomes: These contribute to the formation of the axial skeleton.
• Mesomere or nephrotome: This middle portion of the mesoderm develops into the excretory and genital organs.
• Hypomere or lateral plates: The ventral part of the mesoderm, which divides on either side to form layers with a narrow space between them called the coelom. The outer layer of the mesoderm becomes the somatic layer, while the inner layer forms the visceral layer of the coelom.

c. Formation of endoderm: The cells that form the floor of the archenteron divide and extend dorsally, completely enclosing the archenteron. This layer, located below the mesoderm, becomes the endoderm.

At this stage, the embryo has elongated and consists of the three primary germinal layers: ectoderm on the outer side, endoderm on the inner side, and mesoderm sandwiched between them. These germinal layers will give rise to specific tissues and organs as the embryo continues to develop during organogenesis.

Fate of three germinal layers

During embryonic development, the three primary germinal layers – ectoderm, endoderm, and mesoderm – give rise to specific tissues and organs in living organisms. Here is the fate of each germinal layer:

1. Ectoderm: The ectoderm develops into various structures, including:

• Epidermis (outermost layer of the skin) and cutaneous glands.
• The lining of the cloaca (the common chamber for excretory and reproductive systems) and the mouth cavity.
• Central nervous system, including the brain and spinal cord.
• Lens, cornea, and retina of the eye.
• Olfactory (smell) and auditory (hearing) organs.

2. Endoderm: The endoderm contributes to the formation of several organs and tissues, such as:

• The epithelium lining the digestive canal, except for the mouth and cloaca. It includes the stomach, intestines, and associated structures.
• Digestive glands, such as the liver and pancreas.
• Larynx, trachea, and lungs of the respiratory system.
• The lining of the urinary bladder.
• Thymus and thyroid glands.

3. Mesoderm: The mesoderm plays a crucial role in the development of the following structures:

• Dermis (middle layer) of the skin.
• Cartilage and bones of the skeletal system.
• Blood vascular system, including blood cells and blood vessels.
• Excretory and genital organs, such as kidneys and reproductive organs.
• Spleen, sclera (white part), and choroid (vascular layer) of the eye.

These three germinal layers give rise to a wide range of tissues and organs in living organisms, contributing to the complexity and diversity of their structures and functions.

9. Morphogenesis

Further development of elongated embryo occurs leading to the pre-tadpole stage followed by the larval stage.

a. Pre-tadpole stage:

• The pre-tadpole stage of frog development occurs approximately four days after fertilization. At this stage, the embryo has grown to a length of about 4mm and is enclosed within the protective egg membrane.
• The body of the embryo can be divided into three distinct regions: the head, trunk, and tail. On the head, there are round elevations present on either side, which indicate the future position of the tympanum, an organ involved in hearing.
• On the ventral side of the anterior end of the embryo, there is a U-shaped sucker known as the cement gland. This sucker is formed by mucus gland cells and aids in attachment to surfaces. Between the sucker and the nasal pit, a small depression forms.
• At the posterior end of the embryo, there is another depression called the proctodaeum. This region plays a role in the formation of the anus and the posterior part of the digestive tract.
• As development progresses, the body elongates from the posterior to form the tail. Internally, the embryo contains several important structures, including parts of the central nervous system, the notochord (a flexible rod-like structure that serves as a temporary skeleton), a closed alimentary canal (digestive tract), liver, heart, and a rudimentary form of the urinary bladder.
• Once these organs have developed, the embryo is ready to hatch out of the egg and enter the next stage of its life cycle. The pre-tadpole stage marks an important milestone in the morphogenesis of the frog, as it prepares for further growth and development.

b. Hatching

• Hatching is a significant event in the life cycle of a frog, marking the transition from the embryonic stage to the larval stage. It typically occurs around two weeks after fertilization when the embryo has reached a length of approximately 6mm.
• During hatching, the embryo breaks out of the protective egg membrane and emerges into the surrounding environment. This process allows the developing frog to become free-living and embark on its journey as a tadpole.
• Hatching is a crucial step in the morphogenesis of frogs as it signifies the completion of the embryonic development within the egg. The embryo has grown and developed the necessary structures and organs required for survival outside the egg.
• Once hatching occurs, the newly emerged larval stage of the frog is commonly referred to as a tadpole. Tadpoles are distinct from the embryonic stage as they have a more recognizable form and specialized adaptations for their aquatic lifestyle. They exhibit fish-like characteristics, with a small, blackish body and the ability to swim in water.
• The hatched tadpoles are now equipped to explore their environment, feed, and undergo further development. This marks the beginning of a new phase in the frog’s life cycle, where it will undergo remarkable transformations as it progresses towards adulthood.
• Overall, hatching is a significant milestone in frog development, signifying the transition from the enclosed, embryonic stage to the free-living, larval stage known as tadpoles.

c. Tadpole larva

• The tadpole larva is the stage that follows hatching in the development of a frog. Here is an overview of the characteristics and changes that occur during this phase:
• The newly hatched tadpole appears as a small, blackish creature resembling a fish. It is adapted for an aquatic lifestyle, enabling it to thrive in water.
• Initially, the tadpole attaches itself to an aquatic plant using its ventral sucker or cement gland. This attachment helps the tadpole stay in place and navigate its surroundings.
• During the early stages, two pairs of external gills develop on each side of the tadpole’s head. These gills play a crucial role in respiration, along with the exchange of gases through the tadpole’s skin. As the tadpole grows, these initial gills transform into three pairs of branched gills.
• In the first week after hatching, the tadpole relies on the remaining yolk stored in the cells of the archenteron for nourishment. This yolk provides essential nutrients to support the tadpole’s early growth and development.
• Around seven days after hatching, the tadpole’s mouth begins to form. This mouth is bound by two horny jaws, which enable the tadpole to feed on aquatic plants and other organic matter present in its environment.
• The stomodaeum (anterior part of the digestive tract) and the proctodaeum (posterior part) connect with the gut, forming a complete alimentary canal. Initially, the alimentary canal is small and broad, but it gradually elongates and coils like a spring.
• As the tadpole matures, the three pairs of external gills are gradually replaced by four pairs of internal gills. These internal gills are covered by a protective structure called an operculum.
• The mesonephric kidney, which is the precursor to the adult kidney, begins to appear during this stage. This kidney will develop further as the tadpole transforms into an adult frog.
• The lateral line system, responsible for detecting changes in water movement and pressure, becomes well-developed in the tadpole larva.
• The formation of limbs begins, with both forelimbs and hind limbs starting to develop. However, the forelimbs develop more slowly than the hind limbs as they are initially excluded by the operculum.
• Lungs also start to appear as the tadpole grows. Once fully formed, the tadpole will be able to respire using both its lungs and gills, allowing it to breathe in air as well as extract oxygen from the water.
• The tadpole larva stage is a critical period of growth and transformation for the developing frog, as it acquires structures and adaptations necessary for survival in its aquatic habitat.

10. Metamorphosis and growth

Metamorphosis and growth are critical stages in the life cycle of a frog, marking the transformation from the tadpole larva to the adult frog form. Here are the key features and changes that occur during this process:

The tadpole larva is vastly different from the adult frog in both appearance and nature. After approximately two or three weeks of breathing through its lungs, the tadpole undergoes a remarkable series of changes known as metamorphosis.

Metamorphosis encompasses various morphological, physiological, and behavioral changes that occur during the transition from tadpole to frog.

Prior to metamorphosis, the tadpole develops a thyroid gland, which secretes the hormone thyroxine. This hormone is essential for initiating and regulating the metamorphic changes.

Several significant changes take place during metamorphosis:

1. The tadpole stops feeding as it prepares for the transition into the adult form.
2. The tail tissue develops into a nutritive substance that provides nourishment to the growing larva through the bloodstream.
3. The tail gradually diminishes in size and eventually disappears.
4. The mouth widens and a large, sticky tongue develops, facilitating the capture of prey.
5. The eyes increase in size to adapt to the adult frog’s visual requirements.
6. The forelimbs emerge from the operculum (gill cover), while the hind limbs become longer, facilitating movement on land.
7. The skin undergoes changes, becoming more vascular, glandular, respiratory, and pigmented.
8. The lateral line system, which detects changes in water movement, disappears.

Alongside these external changes, internal transformations also occur:

1. The gills and operculum completely disappear, as the frog transitions to a terrestrial lifestyle.
2. The lungs become more functional as respiratory organs for breathing air.
3. The cartilaginous skeleton of the tadpole is replaced by a bony endoskeleton.
4. The middle ear and tympanum develop, enabling the frog to perceive sound.
5. The stomach and liver enlarge to accommodate the dietary shift from herbivorous to carnivorous.
6. The long, coiled intestine shortens in length.
7. The young frog possesses a stumpy tail.

Following these metamorphic changes, the young frog leaves the water and begins to inhabit damp places on land. It adopts a carnivorous diet, feeding on insects and other small creatures. As it continues to grow, it transforms into an adult frog, leading an amphibious life.

Metamorphosis and growth represent a remarkable and dynamic phase in the life of a frog, enabling it to transition from an aquatic larval form to a terrestrial and carnivorous adult form.

FAQ

References

• https://www.wardsci.com/store/product/8875044/frog-development-slide-set
• http://www.svc.ac.in/SVC_MAIN/Departments/Zoology/Museum/MuseumData/5.%20DEVELOPMENTAL%20BIOLOGY/1_FROG%20EMBRYOLOGY%20SLIDES/5.1%20FROG%20EMBRYOLOGY%20SLIDES.pdf
• http://bcs.whfreeman.com/webpub/biology/sadavalife9e/animated%20tutorials/life9e_3301_script.html
• https://onlinesciencenotes.com/sequential-events-and-stages-in-the-development-of-frog-pre-embryonic-embryonic-and-post-embryonic-development/
• https://old.amu.ac.in/emp/studym/100003759.pdf
• https://www.notesonzoology.com/frog/development-of-frog-with-diagram-vertebrates-chordata-zoology/8626
• https://alrasheedcol.edu.iq/modules/lect/lect/2925-blatolation.pdf
• https://www.yourarticlelibrary.com/science/development-of-frog-stages-in-gastrulating/23140