Development of frog Questions in English

(A) fully grown tadpole larva of a frog is an aquatic stage in its life cycle.
In this stage,the larva possesses internal gills,which are the primary respiratory organs used for gas exchange in water.
As the tadpole undergoes metamorphosis to become an adult frog,the gills are replaced by lungs for terrestrial respiration,and the skin also becomes an important respiratory surface.

(C) In frogs,the egg is released from the ovary at the stage of the secondary oocyte,which is arrested in the metaphase of the second meiotic division. When a sperm enters the egg,it triggers the completion of the second meiotic division,resulting in the formation of a mature ovum and a second polar body.

(D) The correct answer is $D$.
During the process of fertilization,the sperm undergoes an acrosomal reaction upon contact with the egg.
The acrosome releases various hydrolytic enzymes collectively known as sperm lysins.
These enzymes,such as hyaluronidase and acrosin,help in dissolving the protective covering membranes (like the jelly coat and vitelline membrane) of the oocyte,allowing the sperm to penetrate and reach the plasma membrane of the egg.

(A) The acrosome of the sperm contains hydrolytic enzymes collectively known as $Sperm \text{ } lysin$.
These enzymes are essential for the penetration of the sperm through the egg's protective layers (such as the jelly coat and vitelline membrane) during the process of fertilization.
In frogs, this enzyme helps in dissolving the egg membranes to facilitate the entry of the sperm nucleus into the egg cytoplasm.

(B) The correct answer is $B$.
In indirect development,the young ones do not resemble the adults.
Frogs undergo metamorphosis,where the larva (tadpole) passes through intermediate stages before transforming into the adult form.

(A) The correct statement is that the albumen covering the eggs of a frog swells and forms a protective jelly after coming in contact with water.
This jelly layer provides protection against mechanical injury and predators,and also helps in maintaining the moisture content around the developing embryo.
Option $B$ is incorrect because hyaluronidase is present in the acrosome of the sperm,not the egg.
Option $C$ is incorrect because only the head and middle piece of the sperm enter the egg in mammals; the tail is usually left outside.
Option $D$ is incorrect because toads,like most amphibians,typically require water for external fertilization.

(D) In the process of frog oogenesis,the primary oocyte undergoes the first meiotic division (meiosis-$I$).
During this division,the diploid $(2n)$ primary oocyte divides into two unequal haploid $(n)$ daughter cells: a large secondary oocyte and a small first polar body.
Therefore,the reduction of the chromosome number from diploid to haploid occurs when the first polar body is separated.

(C) The title of "father of modern embryology" is attributed to Karl Ernst von Baer $(1792-1876)$.
He is recognized for his significant contributions to the field, including the discovery of the mammalian ovum in $1827$ (often cited as $1827$ in historical records, though the prompt mentions $1872$ which is a typo for the discovery date).
His work established the principles of comparative embryology and the germ layer theory.

(C) In amphibian eggs,the entry of the sperm triggers a cortical rotation of the cytoplasm relative to the inner cytoplasm. This rotation occurs at an angle of approximately $30^{\circ}$ relative to the point of sperm entry. This process leads to the formation of the grey crescent on the side opposite to the point of sperm entry. The grey crescent is a crucial region that marks the site where gastrulation begins and eventually gives rise to the dorsal lip of the blastopore.

(C) In deuterostomes,the blastopore develops into the anus,while the mouth develops secondarily. In the context of chordates like frogs,the blastopore is the opening that eventually forms the anus.

(A) The correct answer is $A$.
In determinate cleavage,the developmental fate of each blastomere is fixed very early,often before cleavage begins.
If a blastomere is removed,the resulting embryo will be incomplete.
This type of development is known as mosaic development because the embryo acts like a mosaic of independent parts.
Examples include organisms like $Nematoda$ and $Mollusca$.

(A) In macro and telolecithal eggs,the yolk is concentrated at the vegetal pole,pushing the active cytoplasm to the animal pole.
Amphimixis,which is the fusion of male and female pronuclei,occurs in the active cytoplasm located at the animal pole because this region contains the germinal vesicle and provides an accessible path for the male pronucleus to reach the female pronucleus.

(D) The number of cells formed after $n$ cleavages is given by the formula $2^n$.
For the fifth cleavage, $n = 5$.
Therefore, the number of cells = $2^5 = 2 \times 2 \times 2 \times 2 \times 2 = 32$ cells.
Thus, the fifth cleavage of the fertilized egg of a frog results in the formation of $32$ blastomeres.

(C) The egg of a frog is classified as $Mesolecithal$.
In $Mesolecithal$ eggs,a moderate amount of yolk is present,which is concentrated at the vegetal pole.
While some classifications refer to it as $Telolecithal$ due to the uneven distribution of yolk,in the context of standard biological classification based on the amount and distribution of yolk,it is specifically termed $Mesolecithal$.

(A) The cleavage in a frog is unequal holoblastic,which occurs in mesolecithal eggs.
This process results in the formation of blastomeres of unequal sizes,known as micromeres and megameres.
Therefore,the correct answer is $A$ (Holoblastic).

(B) The pattern of cleavage in a frog's egg is holoblastic and unequal.
This occurs because the frog's egg is mesolecithal (containing a moderate amount of yolk concentrated at the vegetal pole).
The cleavage furrow passes through the entire egg (holoblastic),but the presence of yolk at the vegetal pole slows down the process,resulting in unequal-sized blastomeres: smaller cells called micromeres at the animal pole and larger cells called macromeres at the vegetal pole.

(A) The egg of a frog is telolecithal and exhibits polarity. It consists of two hemispheres: the animal hemisphere,which is pigmented and contains the nucleus,and the vegetal hemisphere,which is rich in yolk and is typically non-pigmented. Therefore,the vegetal hemisphere consists of yolk.

(B) The egg of a frog is mesolecithal,meaning it contains a moderate amount of yolk. The yolk is not distributed evenly but is concentrated heavily in the vegetal pole. During cleavage,the presence of this dense yolk in the vegetal hemisphere acts as a physical barrier,which slows down the rate of cell division compared to the animal pole. Consequently,the cleavage furrows cannot pass through the vegetal pole as quickly or as completely as they do through the animal pole,resulting in unequal holoblastic cleavage rather than equal holoblastic cleavage.

(C) The correct answer is $C$. In birds (like the chick) and other animals with polylecithal (megalecithal) eggs,the yolk is very abundant and occupies most of the egg volume.
Due to the presence of a large amount of yolk,the cleavage divisions are restricted to a small disc-like area of cytoplasm and the nucleus located at the animal pole.
This type of incomplete cleavage is known as discoidal meroblastic cleavage.

(A) During the process of gastrulation in amphibians,the blastopore is initially open.
As the involution of cells continues,the yolk-filled cells of the vegetal pole are pushed into the blastopore.
These yolk-rich cells protrude through the blastopore,forming a structure known as the yolk plug.
Since these cells originate from the endoderm,the yolk plug is composed of endodermal cells.

(C) . Involution is the process of rolling or turning in of the surface cells into the interior of the embryo. This movement is characteristic of gastrulation and occurs notably in the $frog$ blastula.

(D) In birds (aves),gastrulation is a complex process involving the formation of the primitive streak. Pasteels $(1945)$ specifically described the process of gastrulation in birds as occurring through the mechanism of polyinvagination. This involves the inward migration of cells from the epiblast to form the hypoblast and subsequent layers.

(D) During the process of gastrulation in amphibian embryos,the first external sign is the formation of the dorsal lip of the blastopore. This structure appears at the site of the grey crescent,specifically just behind the grey crescent area,marking the beginning of involution where cells begin to migrate into the interior of the embryo.

(C) During the process of gastrulation in amphibians,the first phase involves the involution of cells at the blastopore lip. The cells that migrate inward first are the prospective endodermal cells. Therefore,the separation and internalization of the endoderm mark the initial phase of gastrulation,followed by the mesoderm and finally the ectoderm remains on the surface.

(C) The concept of the 'organiser' in embryonic development was proposed by Hans Spemann and Hilde Mangold. Hans Spemann of Germany was awarded the Nobel Prize in Physiology or Medicine in $1935$ for his discovery of the 'organiser effect' in embryonic development,which explains how specific regions of the embryo direct the differentiation of surrounding tissues.

(D) Organogenesis is the process in embryology during which the three germ layers (ectoderm,mesoderm,and endoderm) differentiate into the internal organs of an organism.
Since the eye is a complex organ formed from the interaction of different tissues (primarily ectoderm and neural tissue),its development is categorized under organogenesis.
Notogenesis refers to the formation of the notochord.
Neurogenesis refers to the formation of neurons.
Mesogenesis refers to the formation of mesodermal tissues.

(A) In the development of chordate embryos,the egg is typically telolecithal or mesolecithal. The animal pole,which contains the nucleus and less yolk,is metabolically more active. During gastrulation and subsequent morphogenesis,the animal pole region primarily gives rise to the ectoderm,which eventually forms the dorsal structures of the adult,including the nervous system and the dorsal body wall.

(A) The correct statement is $A$.
During blastulation,the cells of the blastoderm undergo rearrangement,and the major presumptive and organ-forming areas are segregated into definite regions.
Option $B$ is incorrect because the three germinal layers are established during gastrulation,not blastulation.
Option $C$ is incorrect because the blastula of a frog is called a coeloblastula,while discoblastula is found in organisms with discoidal cleavage (e.g.,birds).
Option $D$ is incorrect because the fluid-filled space in a blastula is called the blastocoel,whereas the archenteron is the primitive gut cavity formed during gastrulation.

(B) During the process of gastrulation in amphibians,the blastopore is initially occupied by a mass of yolk-filled cells known as the yolk plug.
As development proceeds,the yolk plug is internalized and eventually comes to represent the posterior end of the developing embryo (the region near the anus).

(D) In the development of the frog embryo,the primary organizer is the region that induces the formation of the neural tube and establishes the body axis.
$1$. The $Grey \ crescent$ area,which appears after fertilization,is the precursor region that gives rise to the dorsal lip of the blastopore.
$2$. The $Dorsal \ lip$ of the blastopore (often referred to as the dorsal tip of the gastrula) acts as the primary organizer.
$3$. This region consists of $Chorda-mesoderm$ cells that undergo involution during gastrulation.
Since all these terms describe the same developmental structure at different stages or functional perspectives,the correct answer is $D$.

(D) The development of sexual reproductive organs and the production of offspring by a larval or pre-adult animal is known as $Paedogenesis$ or $Neoteny$. In this phenomenon,the organism attains sexual maturity while retaining larval characteristics.

(B) Metamorphosis is a biological process by which an animal physically develops after birth or hatching,involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.
In the life cycle of a butterfly,the transition from an egg to a larva (caterpillar),then to a pupa,and finally to an adult (imago) represents a classic example of complete metamorphosis.
Option $B$ is the correct answer because the caterpillar undergoes a drastic transformation to become a butterfly.

(C) The thyroid gland in a tadpole secretes the hormone thyroxine. This hormone is essential for the process of metamorphosis,which is the transformation of a tadpole into an adult frog. If the thyroid gland is removed,the tadpole will not receive the necessary thyroxine to trigger metamorphosis and,therefore,will remain in the tadpole stage indefinitely.

(C) Parthenogenesis is a form of asexual reproduction in which an embryo develops from an unfertilized egg cell.
In this process,the egg undergoes cleavage and develops into a new organism without the involvement of fertilization by a sperm.
Therefore,the correct description is that the egg undergoes cleavage without fertilization.

(C) Thiourea is an antithyroid drug that inhibits the synthesis of thyroid hormones. Since thyroid hormones are essential for the process of metamorphosis in frogs,the administration of thiourea effectively inhibits or blocks this developmental transformation.

(D) The egg membranes are classified based on their origin.
$1$. Primary membrane: Secreted by the egg itself (e.g.,vitelline membrane).
$2$. Secondary membrane: Secreted by the follicle cells of the ovary (e.g.,chorion in insects).
$3$. Tertiary membrane: Secreted by the oviduct or other accessory reproductive glands as the egg passes through the reproductive tract.
The jelly layer surrounding the frog's egg is secreted by the oviduct as the egg moves through it,therefore it is classified as a tertiary membrane.

(D) During the metamorphosis of a frog's tadpole, significant anatomical and physiological changes occur to transition from an aquatic, herbivorous larva to a terrestrial, carnivorous adult.
$1$. The $Intestine$ undergoes a drastic reduction in length as the diet shifts from herbivorous (algae) to carnivorous (insects).
$2$. The $Lung$ develops as the animal transitions from gill-based respiration to pulmonary respiration.
$3$. The $Heart$ undergoes structural remodeling to accommodate the shift from a single-circuit circulatory system to a double-circuit system.
$4$. The $Nervous \text{ } system$ remains fundamentally the same in its basic organization, although it undergoes maturation and refinement to support the complex behaviors of the adult frog.

(C) The process of gastrulation in a frog embryo involves the movement of cells to form the three germ layers.
Archenteron formation,which is the primitive gut cavity,begins during the early gastrula stage through the process of invagination at the blastopore.
Therefore,the correct stage is the early gastrula.

(D) Gastrulation is a complex morphogenetic process in the development of a frog embryo that transforms the blastula into a gastrula.
This process involves several coordinated cell movements:
$1$. $Epiboly$: The expansion of the ectodermal cell sheet to cover the entire embryo.
$2$. $Emboly$: The inward movement of cells into the interior of the embryo.
$3$. $Invagination$: A specific type of inward folding of a sheet of cells, which is a key component of $Emboly$ during the formation of the blastopore.
Since all these processes are integral parts of frog gastrulation, the correct answer is $All \text{ of these}$.

(A) During the life cycle of a frog,the larval stage (tadpole) possesses a tail for swimming in an aquatic environment. As the tadpole undergoes metamorphosis to become an adult frog,the tail is gradually reabsorbed and becomes degenerate,as the adult frog develops limbs for terrestrial locomotion.

(B) The blastula stage in the embryonic development of a frog occurs after the morula stage. During this stage,a small,irregular,eccentric cavity appears in the animal hemisphere,which is known as the blastocoel or segmentation cavity. Therefore,the correct answer is $(b)$.

(C) The third cleavage in frog development is horizontal (latitudinal or transverse). It is not equatorial because it occurs decidedly nearer to the animal pole. Consequently,this division results in the formation of $8$ cells.

(A) The embryonic development of a frog follows a specific chronological order:
$1$. Fertilization results in the formation of a single-celled $Zygote$.
$2$. The $Zygote$ undergoes a series of rapid mitotic divisions known as $Cleavage$,which increases the number of cells without increasing the total volume.
$3$. This process leads to the formation of a hollow,spherical structure called the $Blastula$.
$4$. Finally,the $Blastula$ undergoes $Gastrulation$ to form the $Gastrula$,which establishes the three germ layers.

(D) $Neoteny$ is a biological phenomenon where an organism retains its larval or embryonic characteristics even after reaching sexual maturity. This means the adult form of the organism continues to exhibit features typically seen only in the juvenile or larval stages.

(B) The grey crescent is a specialized region of the cytoplasm that appears in the fertilized egg of an amphibian,such as a frog.
It forms in the equatorial zone of the egg,specifically on the side geometrically opposite to the point where the sperm enters the ovum.
This region is crucial for the initiation of gastrulation and the establishment of the dorsal-ventral axis of the embryo.

(B) The blastula of a frog is characterized by a central fluid-filled cavity called the blastocoel. Because the blastula is a hollow sphere of cells,it is classified as a $Coeloblastula$. An $Amphiblastula$ is found in sponges,while a $Stereoblastula$ is a solid blastula found in some other organisms.

(C) In $Frogs$,fertilization is external,meaning it occurs outside the female body in the surrounding water medium. The female releases eggs into the water,and the male releases sperm over them,where fusion takes place. In contrast,$Lizards$,$Rats$,and $Birds$ exhibit internal fertilization,where the fusion of gametes occurs inside the female reproductive tract.

(A) In the development of a frog,the first two cleavages are meridional (vertical) and perpendicular to each other. The third cleavage is horizontal (latitudinal) and equatorial,which separates the animal pole from the vegetal pole. Since the frog egg is mesolecithal,the cleavage is holoblastic (complete) but unequal due to the presence of yolk. Therefore,the third cleavage is holoblastic and equatorial.

(A) The grey crescent is a specialized,crescent-shaped region that appears on the surface of a fertilized frog egg. It forms in the animal hemisphere opposite the point of sperm entry. This region is crucial as it marks the site where the dorsal lip of the blastopore will develop during gastrulation,which eventually gives rise to the notochord and the nervous system.

(B) In the development of a chick embryo,the formation of the primitive streak is a critical event during gastrulation.
This process typically begins around $16$ to $18$ hours of incubation.
By $18$ hours of incubation,the primitive streak is clearly visible as a thickened band of cells along the midline of the epiblast,marking the longitudinal axis of the developing embryo.