Fertilisation and Implantation Questions in English

(D) During fertilization,the male gamete (sperm) and the female gamete (egg) come into contact.
The fusion of their nuclei is facilitated by the attraction between the protoplasts of the egg and the sperm.
This process allows the two haploid nuclei to merge,forming a diploid zygote.

(A) After ejaculation into the vagina,sperms are deposited in the female reproductive tract.
While the lifespan of sperm can vary depending on the environment,they typically survive for about $24$ to $48$ hours ($1-2$ days) within the female reproductive tract,although some may survive up to $5$ days under optimal conditions.
However,in standard biological contexts,the most commonly accepted duration for their functional survival is $1-2$ days.

(C) The sperm penetrates the egg wall in most organisms with the help of $Sperm \text{ } lysin$, which is released from the $Acrosome$ of the sperm. This enzyme helps in dissolving the protective layers of the egg, such as the $Corona \text{ } radiata$ and $Zona \text{ } pellucida$, allowing the sperm to enter the egg cytoplasm.

(A) Fertilizins are chemical substances (glycoproteins) secreted by the mature egg (ovum) to attract sperms.
These substances interact with the antifertilizins present on the surface of the sperms,facilitating the recognition and binding of the sperm to the egg during the process of fertilization.

(C) In mammals,including humans and rabbits,fertilization of the ovum takes place in the fallopian tube (also known as the oviduct or uterine tube). The sperm meets the secondary oocyte in the ampullary region of the fallopian tube,where the fusion of male and female gametes occurs to form a zygote.

(D) During the process of fertilization,when a sperm comes in contact with the $Zona$ $Pellucida$ layer of the ovum,it induces changes in the membrane that block the entry of additional sperms.
Specifically,the cortical granules present beneath the plasma membrane of the ovum release enzymes that harden the $Zona$ $Pellucida$ and modify the vitelline membrane,resulting in the formation of a $Fertilization$ $Membrane$.
This $Fertilization$ $Membrane$ acts as a physical barrier,ensuring that only one sperm fertilizes the ovum,thereby preventing polyspermy.

(B) During fertilization,the entry of the sperm into the secondary oocyte induces the completion of the second meiotic division.
This division is unequal and results in the formation of a second polar body and a haploid ovum (ootid).
Following this,the haploid nucleus of the sperm and the haploid nucleus of the ovum fuse together to form a diploid zygote.
Therefore,the entry of the sperm is the trigger that activates the egg to complete its meiosis.

(D) . Oscar Hertwig $(1875)$ first described the phenomenon of fertilization,which involves the fusion of the sperm nucleus and the egg nucleus,in sea urchins.

(C) The process of fertilization involves several steps,starting from the contact of sperm with the zona pellucida of the ovum.
Entry of the sperm stimulates the secondary oocyte to complete the suspended meiosis-$II$,resulting in the formation of one ootid and a second polar body.
The ootid matures into a functional ovum.
The final event of fertilization is the fusion of the male and female pronuclei,a process known as $Amphimixis$ (or karyogamy),which restores the diploid number of chromosomes in the zygote.

(B) Amphimixis refers to the fusion of the male and female pronuclei during the process of fertilization. After the sperm enters the ovum,the male and female haploid nuclei (pronuclei) approach each other and fuse to form a diploid zygote nucleus. This fusion event is specifically termed as amphimixis.

(A) During the process of fertilization,the sperm enters the egg cytoplasm.
The sperm contains two centrioles: the proximal centriole and the distal centriole.
The proximal centriole is located near the nucleus of the sperm head.
Upon entry into the egg,the proximal centriole organizes the microtubules to form the mitotic spindle apparatus,which is essential for the first cleavage division of the zygote.
The distal centriole is involved in the formation of the sperm tail (axoneme) and does not participate in spindle formation.

(C) During the process of fertilization,when a sperm makes contact with the egg,it triggers the cortical reaction.
Cortical granules,which are present just beneath the plasma membrane of the egg,release their contents into the perivitelline space via exocytosis.
These granules contain enzymes and acid mucopolysaccharides.
The acid mucopolysaccharides absorb water and swell,creating a physical barrier that prevents polyspermy.
This modified structure is known as the fertilization membrane.

(B) During the process of fertilization,only one sperm is allowed to enter the ovum.
When more than one sperm successfully penetrate the egg,this condition is known as Polyspermy.
Polyspermy is generally considered a pathological condition in most animals,as it leads to an abnormal number of chromosomes (polyploidy) in the zygote,which is usually lethal to the developing embryo.

(B) The repressor theory of fertilization,also known as the 'repressor hypothesis',was proposed by $Runnstrom$ in $1949$. According to this theory,the egg cytoplasm contains a repressor substance that inhibits the metabolic activities of the egg. Upon fertilization,the sperm introduces an activator that neutralizes or removes this repressor,thereby initiating the metabolic processes required for embryonic development.

(D) Fertilizin is a glycoprotein secreted by the mature egg (ovum).
It plays a crucial role in the process of fertilization by interacting with antifertilizin present on the surface of the sperm.
This interaction results in the following:
$1$. It makes the sperm sticky,facilitating their attachment to the egg surface.
$2$. It causes the clumping (agglutination) of sperms,which helps in selecting the most compatible sperm.
$3$. It ensures the sperm adheres firmly to the egg's zona pellucida,allowing for successful penetration.
Therefore,all the given options are correct functions of fertilizin.

(A) Gynogamones are chemical substances secreted by the female gamete (ovum or unfertilized egg). These substances play a crucial role in the process of fertilization by attracting the male gametes (spermatozoa) towards the ovum. There are two types of gynogamones: $Gynogamone-I$ (which attracts sperm) and $Gynogamone-II$ (which activates sperm).

(A) Antifertilizin molecules are present on the surface of spermatozoa. These are acidic proteins with relatively small molecular weight. During fertilization,the antifertilizin of the sperm reacts with the fertilizin present on the surface of the egg (ovum) to facilitate the attachment of the sperm to the egg.

(A) Fertilizin is a glycoprotein substance secreted by the mature ovum (egg cell). It acts as a chemical attractant for sperm,facilitating the process of fertilization by interacting with the antifertilizin present on the surface of the sperm. Therefore,it is present in the mature ovum.

(A) Immediately upon the entry of a spermatozoan into an oocyte,the oocyte membrane undergoes a rapid change in its electrical potential,known as depolarization. This change is associated with an increase in the permeability of the oocyte membrane to certain ions,which serves as a fast block to polyspermy. Therefore,the correct event is an increase in the permeability of the oocyte membrane.

(D) In eggs with very thick and resistant membranes,such as those of fishes and insects,the sperm cannot penetrate the membrane directly. Instead,the sperm enters through a specialized small opening or canal in the egg membrane known as the $Micropyle$. This structure ensures that the sperm can reach the egg cytoplasm for fertilization.

(B) The acrosome reaction in the hemichordate $Saccoglossus$ was extensively studied and demonstrated by the researchers $A$.$L$. Colwin and $L$.$H$. Colwin in $1967$. Their work provided detailed electron microscopic evidence of the fusion between the acrosomal membrane and the plasma membrane during the fertilization process.

(C) Cortical granules are specialized secretory vesicles located in the cortex of the egg,specifically situated just beneath the plasma membrane. Upon fertilization,these granules undergo exocytosis,releasing their contents into the perivitelline space. This process modifies the zona pellucida and the plasma membrane to prevent polyspermy,a mechanism known as the cortical reaction.

(C) Fertilization involves chemical interactions between the sperm and the egg. These chemicals are known as gamones.
$Androgamones$ are secreted by the sperm,while $Gynogamones$ are secreted by the egg.
$Androgamone-I$ is known to inhibit sperm motility to conserve energy.
$Gynogamone-I$ acts by neutralizing $Androgamone-I$,which releases the inhibition on the sperm and thereby increases sperm activity and motility as they approach the egg.

(C) In sexually reproducing organisms,the process of fusion of a male gamete (sperm) and a female gamete (ovum) is known as fertilization.
This fusion results in the formation of a single diploid cell called the $Zygote$.
The $Zygote$ represents the beginning of the life of a new individual.

(A) Fertilizin is a chemical substance secreted by the mature egg (specifically the jelly coat or vitelline membrane).
It is a glycoprotein or mucopolysaccharide in nature.
Its primary function is to attract and facilitate the binding of compatible sperm to the egg surface during the process of fertilization.

(B) The correct answer is $B$. The sperm penetrates the ovum primarily through chemical means.
Specifically,the acrosome of the sperm releases enzymes,such as hyaluronidase,which digest the hyaluronic acid in the corona radiata and the zona pellucida of the ovum.
This enzymatic action allows the sperm to pass through the protective layers of the egg,facilitating fertilization.

(D) Determinate cleavage (also known as mosaic cleavage) is a type of embryonic development where the fate of each blastomere is determined very early.
In this process,the orientation of the mitotic spindle is typically $Oblique$ relative to the animal-vegetal axis of the egg.
This specific orientation leads to unequal distribution of cytoplasmic determinants,which restricts the developmental potential of the resulting blastomeres.

(C) After fertilization,the zygote undergoes a series of rapid mitotic cell divisions known as $Cleavage$.
These divisions occur without significant growth of the cells,resulting in the formation of a multicellular structure called a $Blastocyst$.

(D) In $Holoblastic$ (total or complete) cleavage,the entire egg divides by each cleavage furrow.
This type of cleavage occurs in eggs with little or no yolk.
It can be further classified as $(i)$ equal $Holoblastic$ or $(ii)$ unequal $Holoblastic$ cleavage.

(B) Cleavage refers to the series of rapid mitotic cell divisions of the zygote following fertilization.
During this process,the total volume of the embryo remains constant because there is no growth phase (interphase) between the divisions.
The cells produced by these divisions are called blastomeres.
Since the cells divide repeatedly without increasing in total cytoplasmic mass,the size of individual blastomeres decreases with each successive division.

(B) . Cleavage in the fertilized egg (zygote) in humans is holoblastic and occurs in the fallopian tube (oviduct) as the zygote moves towards the uterus. Unlike normal mitosis,cleavage involves rapid cell divisions without significant growth between divisions,resulting in smaller cells called blastomeres.

(B) The first cleavage is meridional and results in $2$ blastomeres.
The second cleavage is also meridional but at a right angle to the first one,resulting in $4$ blastomeres.
These $4$ blastomeres are identical with respect to cytoplasm,pigment,and yolk gradient.

(C) Cleavage is a series of rapid mitotic divisions of the zygote that occur without any intervening growth phases ($G_1$ and $G_2$ phases).
In normal mitosis,cells undergo growth phases to increase their size and mass before dividing.
In contrast,during cleavage,the total volume of the embryo remains constant while the number of cells (blastomeres) increases,meaning the cells become progressively smaller with each division.

(C) In teleosts,reptiles,and birds,the eggs are polylecithal (containing a large amount of yolk).
Due to the presence of this enormous amount of yolk,the cleavage is restricted to a small portion of the cytoplasm,which is known as meroblastic cleavage.
Specifically,in these organisms,it is discoidal meroblastic cleavage,which is a type of incomplete meroblastic cleavage.

(A) The cleavage plane that passes through the animal and vegetal poles is called a meridional cleavage plane.
If the cleavage furrow passes through the animal and vegetal poles but does not divide the egg into two equal halves,it is still considered a meridional cleavage,specifically an unequal meridional cleavage.
In contrast,an equatorial cleavage plane passes through the equator of the egg,perpendicular to the animal-vegetal axis.

(D) In radial cleavage,the cleavage planes are either parallel or at right angles to the polar axis of the egg. This results in blastomeres being arranged symmetrically around the polar axis,forming tiers of cells directly above one another. This type of cleavage is characteristic of echinoderms and some chordates.

(A) The process of cleavage was first observed by Swammerdam in $1738$ in the eggs of a frog. Cleavage refers to the series of rapid mitotic cell divisions of the zygote that occur immediately after fertilization.

(C) The $Zona$ $pellucida$ is a glycoprotein layer surrounding the plasma membrane of a mammalian oocyte.
It plays a crucial role in fertilization by acting as a species-specific barrier for sperm binding.
Once a single sperm penetrates the $Zona$ $pellucida$ and fuses with the egg membrane,cortical granules release enzymes that modify the $Zona$ $pellucida$ (the zona reaction),which prevents other sperm from binding or penetrating,thereby preventing polyspermy.
It also protects the developing embryo during its transport through the oviduct and prevents premature implantation.

(A) Cleavage is a series of rapid mitotic cell divisions of the zygote that occurs immediately after fertilization.
It transforms the single-celled zygote into a multicellular structure.
The sequence of development is as follows:
$Zygote \xrightarrow{\text{Cleavage}} \text{Morula} \xrightarrow{\text{Blastulation}} \text{Blastula} \xrightarrow{\text{Gastrulation}} \text{Gastrula}$.

(C) The term $blastocyst$ is specifically used for the $blastula$ stage in placental mammals.
In mammals,as the amount of nutritive fluid increases in the $blastocoel$,the $morula$ enlarges and takes the form of a cyst,which is then called a $blastocyst$.
Examples of organisms that form a $blastocyst$ include humans and monkeys.
Kangaroos and platypuses are not placental mammals (they are marsupials and monotremes,respectively) and do not form a $blastocyst$ in the same manner.

(C) The blastocyst is a structure formed during early embryonic development in mammals. It consists of an outer layer of cells called the $Trophoblast$ and an inner cell mass. The $Trophoblast$ layer attaches to the endometrium of the uterus and eventually forms the embryonic part of the placenta. Therefore,the correct answer is $C$.

(A) Meroblastic cleavage is a type of cleavage in which the division of the zygote is restricted to a small portion of the cytoplasm due to the presence of a large amount of yolk.
It is also known as partial cleavage because the cleavage furrows do not extend through the entire egg.
In contrast,holoblastic cleavage refers to complete cleavage where the entire zygote divides.

(D) The process of pregnancy begins with the implantation of the $Blastocyst$ into the uterine wall (endometrium).
After fertilization,the zygote undergoes cleavage to form a morula and subsequently develops into a $Blastocyst$.
The $Blastocyst$ consists of an outer layer of cells called the trophoblast and an inner cell mass.
Implantation typically occurs about $6$ to $7$ days after fertilization.

(D) Cleavage is a series of rapid mitotic divisions of the zygote that converts the single-celled zygote into a multicellular structure called the $Blastula$ (or $Blastocyst$ in mammals).
Therefore,the process of cleavage results in the formation of the $Blastula$ stage.

(C) . Cleavage is a series of rapid mitotic cell divisions of the zygote that convert the single-celled zygote into a multicellular structure called a blastula.
During cleavage,the total volume of the embryo remains constant,and there is no significant growth in the size of the cells; instead,the cytoplasm is partitioned into smaller cells called blastomeres.
Option $A$ is incorrect because cleavage precedes gastrulation.
Option $B$ is incorrect because the amount and distribution of yolk significantly influence the pattern and rate of cleavage.
Option $D$ is incorrect because blastulation occurs before gastrulation.

(D) Implantation is the process in which the blastocyst attaches itself to the endometrium of the uterus. This process is essential for the further development of the embryo in viviparous animals. Therefore,the correct option is $D$.

(D) The correct answer is $(d)$. Implantation is the process where the blastocyst attaches to the uterine wall. This process typically occurs approximately $7$ days after fertilization.

(C) The peripheral cells of the morula differentiate into the trophoblast (or trophoectoderm) layer.
These cells are responsible for the formation of the four extra-embryonic membranes: amnion,chorion,allantois,and yolk sac.
Therefore,the correct option is $C$.

(B) The $Endometrium$ is the innermost glandular layer of the uterus.
During implantation,the blastocyst attaches to the $Endometrium$.
The chorionic villi (placental villi) grow into the uterine tissue,causing erosion of the $Endometrium$ to establish a connection between the fetal and maternal blood supply for the exchange of nutrients and gases.

(D) In mammals,the placenta is a specialized organ that connects the developing fetus to the uterine wall. It is essential for the effective reproduction of mammals because it facilitates the exchange of nutrients,oxygen,and waste products between the mother and the fetus. This allows for the internal development of the fetus,providing protection and a stable environment,which significantly increases the survival rate of the offspring.