Development of frog Questions in English

(C) In amphibian fertilization,the entry of the sperm triggers a cortical rotation of the egg cytoplasm. The point of sperm entry determines the site where the $Grey \ crescent$ is formed,which is located on the side opposite to the point of sperm entry. This region is crucial for the establishment of the embryonic axis and the subsequent development of the dorsal lip of the blastopore.

(B) During the process of cleavage in many organisms,the zygote undergoes rapid mitotic divisions to form smaller cells called blastomeres.
$1$. Micromeres are small cells that divide rapidly.
$2$. Megameres (also known as macromeres) are larger cells that contain more yolk and divide slowly compared to micromeres.
$3$. Therefore,megameres are the set of cells that divide slowly.

(C) During the process of gastrulation,the middle gastrula stage is marked by the presence of both the $blastopore$ and the $archenteron$. The $archenteron$ is the primitive gut cavity formed during gastrulation,and the $blastopore$ is the opening that connects the $archenteron$ to the exterior. While the yolk plug is a feature of the late gastrula stage,the presence of the $archenteron$ and $blastopore$ is the defining characteristic of the middle gastrula.

(D) The development of the neural tube is determined by the process of embryonic induction. The ectoderm in the neural tissue area is 'determined' to form the neural tube. When this tissue is transplanted to the presumptive belly region (which is normally destined to form the epidermis),it retains its original fate due to its determination. Therefore,the transplanted tissue will form a second neural tube in the belly region,resulting in an embryo with two neural tubes.

(D) This experiment is known as the Spemann-Mangold organizer experiment.
The dorsal lip of the blastopore acts as the primary embryonic organizer.
When this tissue is transplanted to a new location (like the ventral side) in another gastrula,it induces the surrounding host tissue to form a secondary embryonic axis.
This results in the formation of a second notochord and neural tube,leading to the development of a secondary embryo (a Siamese twin) attached to the host.

(B) Mesolecithal eggs contain a moderate amount of yolk,which is concentrated towards the vegetal pole. Due to this moderate yolk distribution,the cleavage is holoblastic (complete) but unequal. The presence of yolk at the vegetal pole slows down the cleavage process in that region,resulting in smaller cells (micromeres) at the animal pole and larger cells (macromeres) at the vegetal pole. Therefore,the cleavage is unequal holoblastic.

(B) fate map is a method used in developmental biology to map larval or adult structures onto the region of the embryo from which they arose.
This map is typically prepared at the blastula stage of embryonic development.
At the blastula stage,the cells are arranged in a way that their future developmental pathways (fates) can be traced to specific regions of the embryo before the complex movements of gastrulation begin.

(B) In oogenesis,one secondary oocyte undergoes meiosis-$II$ to produce one functional ovum and one polar body.
Therefore,$50$ secondary oocytes will produce $50$ ova.
In spermatogenesis,one secondary spermatocyte undergoes meiosis-$II$ to produce two spermatids,which then differentiate into two spermatozoa.
Therefore,$50$ secondary spermatocytes will produce $50 \times 2 = 100$ spermatozoa.
Thus,the correct answer is $50$ ova and $100$ spermatozoa.

(B) Frog eggs are classified as $Mesolecithal$.
In $Mesolecithal$ eggs,there is a moderate amount of yolk present,which is distributed unevenly,typically concentrated towards the vegetal pole.
This moderate amount of yolk is characteristic of amphibian eggs like those of a frog.

(A) In all vertebrates,including frogs,the nervous system originates from the $Ectoderm$. During embryonic development,the neural plate forms from the ectoderm,which then folds to form the neural tube. This neural tube eventually develops into the central nervous system (brain and spinal cord),while the neural crest cells,also derived from the ectoderm,give rise to the peripheral nervous system,including the ganglia.

(B) Meroblastic cleavage is a type of cleavage where the division is partial or incomplete.
This occurs in eggs with a large amount of yolk,such as those of birds,reptiles,and many fishes.
Because the yolk is dense,the cleavage furrow cannot penetrate the entire egg,resulting in division restricted to a small portion of the cytoplasm.

(A) Karl Ernst von Baer is widely recognized as the father of modern embryology. He is famous for discovering the blastula stage of development and the mammalian ovum. He also formulated the laws of embryology,known as Von Baer's laws.

(A) In the development of a frog (a deuterostome),the blastopore is the opening that forms during gastrulation. In deuterostomes,the blastopore develops into the anus,while the mouth forms later at a different location. Therefore,the correct answer is $A$.

(B) Meroblastic cleavage is a type of incomplete cleavage that occurs in eggs with a large amount of yolk (macrolecithal eggs).
In birds,such as the owl,the yolk is abundant,restricting the cleavage process to a small disc of cytoplasm at the animal pole.
This is known as discoidal meroblastic cleavage.
In contrast,mammals like dogs,monkeys,and rhinoceroses have microlecithal eggs with very little yolk,which undergo holoblastic (complete) cleavage.

(B) $Bidder's$ canal is a specialized structure found in the kidneys of male frogs. It is formed by the fusion of $vasa$ $efferentia$ with the longitudinal collecting duct of the kidney. This canal helps in the transport of sperms from the testes to the $urinogenital$ duct.

(B) The egg of a frog is classified as $Mesolecithal$ because it contains a moderate amount of yolk distributed unevenly.
Specifically, the yolk is concentrated more towards the vegetal pole, while the cytoplasm is concentrated at the animal pole.
This type of distribution is also referred to as $Telolecithal$ in some contexts, but $Mesolecithal$ is the most precise term for amphibian eggs due to the moderate quantity of yolk.

(B) The blastopore is the primary opening that forms during gastrulation in the developing embryo.
It serves as the opening of the archenteron,which is the primitive gut cavity.
In protostomes,the blastopore develops into the mouth,while in deuterostomes,it develops into the anus.

(D) Mesolecithal eggs contain a moderate amount of yolk,which is concentrated towards the vegetal pole. Due to this distribution of yolk,the cleavage is unequal holoblastic. In this process,the cleavage furrow passes through the entire egg,but the blastomeres formed at the animal pole (micromeres) are smaller than those at the vegetal pole (macromeres).

(A) During the process of gastrulation in animal development,the archenteron (primitive gut) is formed as the cells invaginate into the blastocoel. As the archenteron expands,it gradually occupies the space previously held by the blastocoel. Therefore,the completion of the formation of the archenteron signifies the complete obliteration (disappearance) of the blastocoel.

(C) Cleavage is the series of mitotic divisions of the zygote.
In animals with telolecithal eggs (eggs with a large amount of yolk concentrated at the vegetal pole),such as birds (e.g.,hen) and reptiles,the yolk inhibits the cleavage furrow from passing through the entire egg.
Consequently,cleavage is restricted to a small disc-like area at the animal pole,which is known as discoidal meroblastic cleavage.
Therefore,in a hen's egg,cleavage is restricted to a small portion of the animal pole.

(A) Frogs are $Oviparous$ animals.
This means they lay eggs in water,and their development is external.
The eggs hatch into larvae called tadpoles,which eventually undergo metamorphosis to become adult frogs.
Therefore,the correct option is $A$.

(A) The metamorphosis of a frog tadpole into an adult frog is regulated by the thyroid hormone,$Thyroxine$.
If the thyroid gland is removed or if the production of $Thyroxine$ is inhibited by anti-thyroid substances like $Thiourea$,the tadpole fails to undergo metamorphosis.
However,the tadpole continues to grow in size because its growth hormone remains active,eventually resulting in a giant tadpole.

(B) During embryonic development,the process by which cells in a developing embryo acquire positional information and organize themselves into specific structures along defined axes is known as $Pattern \ formation$. This process ensures that the embryo develops the correct body plan,establishing the $anterior/posterior$,$dorsal/ventral$,and $medial/lateral$ axes.

(B) The $Gray \text{ } Crescent$ is a surface region of the amphibian egg that appears after fertilization.
It forms on the side of the egg directly opposite to the point of sperm entry.
This region is crucial for the development of the embryo as it marks the site where gastrulation begins and contains determinants for the formation of the dorsal structures of the embryo.

(D) Polyspermy is the fertilization of an egg by more than one sperm.
In most animals,such as mammals,polyspermy is prevented by the cortical reaction,which modifies the zona pellucida.
However,in certain species like birds and some amphibians (e.g.,frogs),physiological polyspermy is a normal developmental process where multiple sperm enter the egg,but only one sperm nucleus fuses with the egg nucleus.
Among the given options,frogs are known to exhibit physiological polyspermy.

(A) The embryonic development of a frog follows a specific sequence of stages:
$1$. $Zygote$: The initial single-celled stage formed after fertilization.
$2$. $Cleavage$: $A$ series of rapid mitotic cell divisions that transform the zygote into a multicellular structure.
$3$. $Blastula$: The stage where the embryo forms a hollow sphere of cells called a blastula.
$4$. $Gastrula$: The stage where the blastula undergoes rearrangement to form the three germ layers.
Therefore,the correct sequence is $Zygote - Cleavage - Blastula - Gastrula$.

(A) The changes that occur during the transformation of a larva into an adult are collectively called metamorphosis.
During the metamorphosis of a frog,the tail disappears,horny jaws are replaced by bony jaws,gills disappear,and lungs become functional.
Thyroxine hormone,which requires iodine for its synthesis,is essential for the metamorphosis of a frog. It accelerates the process of development from the tadpole stage to the adult frog.

(B) Epiboly is a morphogenetic movement during gastrulation where cells (micromeres) divide rapidly and spread downward to cover the larger cells (megameres) of the embryo,except at the yolk plug region.
Invagination involves the inward folding of a sheet of cells into the blastocoel.
Emboly refers to the movement of cells into the interior of the embryo,often involving involution or ingression.

(A) The gray crescent is a surface feature of the amphibian embryo,specifically in frogs,formed during fertilization.
When the sperm enters the egg in the animal hemisphere,the cortical cytoplasm rotates relative to the inner cytoplasm.
This rotation causes the dark pigment granules (melanin) to shift away from the site of sperm entry,exposing the lighter-colored cytoplasm underneath.
This exposed area appears as a gray-colored crescent,known as the gray crescent,located exactly opposite to the point of sperm entry.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the formation of the gray crescent.

(B) In the development of a frog's egg,the cleavage pattern is holoblastic and unequal.
$1$. The first two cleavages are meridional and perpendicular to each other.
$2$. The third cleavage is latitudinal (horizontal) and slightly above the equator.
$3$. The fourth cleavage consists of two simultaneous meridional planes,one in each of the four blastomeres.
Therefore,the fourth cleavage plane is meridional.

(A) During the early stages of embryonic development in frogs,the process of cleavage occurs.
Cleavage involves rapid mitotic cell divisions without any significant growth in the total volume of the embryo.
As the zygote divides into smaller cells called blastomeres,the overall size of the embryo remains constant because the total cytoplasmic volume does not increase until the feeding stage begins.
Therefore,the size of the fertilized egg (zygote),the blastula,and the gastrula remains approximately the same.