Pregnancy and Embryonic development Questions in English

(A) In $Ovo-viviparous$ animals,the embryo develops inside the mother's body,but it is nourished by the yolk of the egg rather than directly from the mother's body through a placenta.
This is a reproductive strategy that combines features of both oviparity and viviparity.
Therefore,the correct option is $A$.

(D) The $Allantois$ is an extra-embryonic membrane that serves as the embryonic urinary bladder in many amniotes,including mammals. It acts as a reservoir for nitrogenous waste products produced by the developing embryo before the placenta takes over the function of waste removal.

(A) After fertilization,the developing embryo implants in the uterus and begins to secrete $hCG$ (human Chorionic Gonadotropin).
This hormone maintains the corpus luteum,which continues to produce high levels of progesterone and estrogen.
These high levels of estrogen and progesterone exert negative feedback on the anterior pituitary gland and the hypothalamus.
This negative feedback inhibits the secretion of $FSH$ (Follicle Stimulating Hormone) and $LH$ (Luteinizing Hormone),thereby preventing the development of new ovarian follicles during pregnancy.

(D) Progesterone is a steroid hormone secreted by the corpus luteum in the ovary.
It plays a crucial role in the maintenance of the endometrium,which is essential for the implantation of the fertilized ovum and the successful progression of pregnancy.
Therefore,it is often referred to as the pregnancy hormone.

(B) Progesterone is a steroid hormone secreted by the corpus luteum in the ovary after ovulation.
It is essential for the maintenance of the endometrium,which is necessary for the implantation of the blastocyst (embryo) in the uterus.
Furthermore,progesterone plays a crucial role in the formation and maintenance of the placenta during pregnancy,ensuring the successful development of the fetus.

(C) The $Placenta$ is a temporary endocrine gland that forms during pregnancy. It secretes several hormones essential for the maintenance of pregnancy, such as $Human \text{ } Chorionic \text{ } Gonadotropin \text{ } (hCG)$, $Human \text{ } Placental \text{ } Lactogen \text{ } (hPL)$, estrogens, and progestogens. Once the pregnancy ends, the placenta is expelled, and its endocrine function ceases.

(C) After fertilization, the human chorionic gonadotropin $(hCG)$ hormone is secreted by the developing placenta. This hormone maintains the corpus luteum, preventing its degeneration. The corpus luteum persists for approximately the first $3$ to $4$ months of pregnancy (about $90$ to $120$ days) to produce progesterone, which is essential for maintaining the endometrium. However, among the given options, $280$ days is the closest approximation to the full duration of pregnancy (gestation period), during which the corpus luteum's function is eventually taken over by the placenta.

(A) $Relaxin$ is a proteinaceous hormone secreted by the corpus luteum (and placenta) towards the end of the gestation period.
It acts to soften the pubic symphysis and ligaments of the pelvic girdle,which facilitates the process of parturition (childbirth).

(B) The correct answer is $B$. During pregnancy,the hormone $Progesterone$ plays a crucial role in the development of the mammary glands. It stimulates the proliferation of the alveolar tissue in the mammary glands,which leads to the enlargement of the breasts in preparation for lactation.

(D) During pregnancy,the placenta produces a hormone called Human Chorionic Gonadotropin $(hCG)$.
This hormone is excreted in the urine of the pregnant woman.
The presence of $hCG$ in the urine is the basis for pregnancy diagnostic tests.

(B) In mammals,the hormone $Relaxin$ is primarily secreted by the $Placenta$ during the later stages of pregnancy. It plays a crucial role in facilitating childbirth by relaxing the pelvic ligaments and softening the cervix.

(C) $Oxytocin$ is a hormone that stimulates the contraction of smooth muscles,particularly those of the uterus. During pregnancy,an abnormally high level of $oxytocin$ can induce premature uterine contractions,which may lead to the expulsion of the fetus,commonly known as an abortion or miscarriage.

(C) The placenta acts as an endocrine tissue and produces several hormones essential for maintaining pregnancy.
These hormones include Human chorionic gonadotropin $(hCG)$,Human placental lactogen $(hPL)$,estrogens,progesterone,and relaxin.
Therefore,the correct option is $(c)$.

(C) . $Progesterone$ is a hormone produced by the mammalian corpus luteum that maintains pregnancy.
It is responsible for the implantation of the embryo,the formation of the uterine bed and placenta,and it also inhibits ovulation and the action of oxytocin on the uterus during pregnancy.

(C) The placenta acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin $(hCG)$,human placental lactogen $(hPL)$,estrogens,and progestogens. Progesterone is essential for the maintenance of pregnancy as it inhibits uterine contractions and supports the development of the endometrium.

(A) During pregnancy,the developing embryo secretes Human Chorionic Gonadotropin $(hCG)$.
This hormone acts on the corpus luteum to maintain its structure and function.
Consequently,the corpus luteum continues to secrete progesterone,which is essential for maintaining the endometrium and supporting the pregnancy.

(B) After ovulation,the ruptured follicle transforms into the corpus luteum under the influence of $LH$.
If fertilization does not occur,the corpus luteum cannot be maintained because the human chorionic gonadotropin $(hCG)$ hormone is absent.
Consequently,the corpus luteum degenerates into a mass of fibrous tissue called the corpus albicans within $10-14$ days.
This degeneration leads to a sharp decline in progesterone and estrogen levels,which triggers the shedding of the uterine endometrium,resulting in menstruation.

(B) $Corpus$ $luteum$ literally means yellow body.
The luteal cells secrete small amounts of estradiol hormone and significant amounts of the progesterone hormone.
$Corpus$ $luteum$ also secretes the hormone relaxin.

(B) During pregnancy,the high levels of progesterone produced initially by the corpus luteum and subsequently by the placenta exert a negative feedback effect on the anterior pituitary gland. This inhibition suppresses the secretion of gonadotropins,specifically Follicle Stimulating Hormone $(FSH)$ and Luteinizing Hormone $(LH)$. Since $FSH$ is required for follicular development and $LH$ is required for ovulation,their suppression prevents further ovulation during the gestation period.

(A) The $corpus$ $luteum$ is a temporary endocrine structure in the ovary that develops from the ruptured follicle after ovulation.
It primarily secretes the hormone $progesterone$,which is essential for maintaining the endometrium of the uterus during pregnancy.
This secretion occurs under the influence of the Luteinizing Hormone $(LH)$.

(B) Teratology is the branch of embryology that deals with the study of abnormalities of physiological development,specifically congenital malformations and birth defects in the embryo or fetus.

(A) The embryonic stage is the period of development from fertilization until the end of the $8^{th}$ week of gestation. During this phase,the fundamental body plan of the organism is established,including the formation of the three germ layers (ectoderm,mesoderm,and endoderm) and the initiation of organogenesis. Therefore,the emergence of the basic body plan is the defining characteristic of this stage.

(A) In mammals,the egg is alecithal (containing very little or no yolk). Due to the absence of a significant amount of yolk,the cleavage furrow passes through the entire egg,dividing it completely. This type of complete cleavage is known as holoblastic cleavage. Specifically,mammalian cleavage is holoblastic and rotational.

(B) Cleavage is a series of mitotic divisions of the zygote. The number of cells produced after $n$ cleavages is given by the formula $2^n$,where $n$ is the number of cleavages.
For the $16$-celled stage,we set $2^n = 16$.
Since $2^4 = 16$,it follows that $n = 4$.
Therefore,$4$ cleavages are required to reach the $16$-celled stage.

(B) The segmentation cavity is also known as the blastocoel. Its formation is a characteristic feature of the blastula stage in embryonic development,where the embryo consists of a hollow sphere of cells.

(B) Epiboly is a morphogenetic movement during gastrulation in which a sheet of cells (micromeres) spreads over the surface of the embryo.
In this process,the micromeres divide rapidly and migrate downwards to cover the larger cells (megameres) of the vegetal hemisphere,except for a small area known as the yolk plug.
Therefore,option $B$ is the correct description of epiboly.

(B) During cleavage,the zygote undergoes rapid mitotic divisions without any significant growth in size. As a result,the total mass of the protoplasm does not increase; instead,the cytoplasm is partitioned into smaller cells called blastomeres.
$1$. Cleavage does not increase the mass of protoplasm; it decreases the size of individual blastomeres.
$2$. In the second cleavage,the division is asynchronous,meaning one blastomere divides slightly earlier than the other.
$3$. With each division,the blastomeres become progressively smaller.
$4$. The morula is a solid ball of cells (typically $8-16$ cells),not a hollow one; the hollow structure is known as the blastocyst.

(C) The blastula is an early stage of embryonic development. It is a hollow sphere of cells formed during the process of cleavage. These individual cells that constitute the blastula are collectively called blastomeres.

(A) At the beginning of cleavage,the zygote contains a large amount of cytoplasm relative to the size of the nucleus,resulting in a very low nucleus-to-cytoplasm ratio.
As cleavage progresses,the cells divide without increasing the total volume of the embryo,which leads to a gradual increase in the number of nuclei until the ratio reaches the level found in typical somatic cells.

(C) The avian blastula is known as a $Discoblastula$ because it is formed through meroblastic discoidal cleavage,which is characteristic of birds,reptiles,fishes,and egg-laying mammals. In these organisms,the cleavage is restricted to a small disc of cytoplasm at the animal pole. This resulting disc-shaped structure is referred to as the $Blastoderm$.

(B) The blastodisc (or germinal disc) is a small,circular,white spot on the surface of a yolk-rich egg where the cytoplasm is concentrated and cleavage occurs.
In telolecithal eggs,such as those found in birds (avian eggs),the massive amount of yolk restricts the active cytoplasm to a small disc-like area at the animal pole,known as the blastodisc.
Therefore,the blastodisc is characteristic of avian eggs.

(B) The phenomenon of embryonic induction,specifically the theory of the organizer (inductor) in amphibia,was first described by Hans Spemann in $1938$.
He was awarded the Nobel Prize in Physiology or Medicine in $1935$ for his discovery of the organizer effect in embryonic development.

(D) Cleavage is a series of rapid mitotic divisions of the zygote.
Following fertilization,the zygote undergoes cleavage to form a solid ball of cells known as the morula ($8-16$ celled stage).
The morula further divides to form a hollow,fluid-filled structure known as the blastula (or blastocyst in mammals).
Since both morula and blastula are stages formed during the process of cleavage,the correct answer is $D$.

(D) The $blastula$ is an early embryonic stage characterized by a hollow sphere of cells called $blastomeres$ surrounding a fluid-filled cavity known as the $blastocoel$. The outer layer of cells is often referred to as the $blastoderm$. $A$ $blastopore$ is the opening of the central cavity $(archenteron)$ of an embryo in the early stages of development,which is a characteristic feature of the $gastrula$ stage,not the $blastula$ stage.

(A) The developmental stages of a mammal follow a specific sequence:
$1$. $Gametogenesis$: The formation of gametes (sperm and egg).
$2$. $Fertilization$: The fusion of the male and female gametes to form a zygote.
$3$. $Cleavage$: The rapid mitotic division of the zygote into a multicellular structure (morula/blastula).
$4$. $Gastrulation$: The process where the blastula is reorganized into a three-layered structure called the gastrula.
Therefore,the third phase in the development is $Cleavage$.

(A) The process of human development begins with fertilization,where the sperm and egg fuse to form a single-celled zygote.
Following fertilization,the zygote undergoes a series of rapid mitotic divisions known as cleavage,which increases the number of cells without increasing the total volume.
This results in the formation of a solid ball of cells called the morula.
The morula then develops into a hollow,fluid-filled structure known as the blastula (or blastocyst in mammals).
Finally,the blastula undergoes gastrulation to form the gastrula,which establishes the three primary germ layers.
Therefore,the correct sequence is: Fertilization $\rightarrow$ Zygote $\rightarrow$ Cleavage $\rightarrow$ Morula $\rightarrow$ Blastula $\rightarrow$ Gastrula.

(B) In indeterminate cleavage,the developmental potential of blastomeres is not fixed at early stages. The fate of these cells is determined during the process of gastrulation,where cell movements and interactions establish the specific germ layers and their future roles in the embryo. Therefore,the correct stage where the fate becomes fixed is gastrulation.

(B) Pseudopregnancy (also known as false pregnancy) is a condition where an individual exhibits the physical signs and symptoms of pregnancy,such as abdominal enlargement,breast changes,and hormonal shifts,without the presence of a developing foetus. In a normal pregnancy,the $Corpus \ luteum$ persists to maintain the endometrium for the developing embryo. In pseudopregnancy,while hormonal changes and endometrial modifications may occur,the essential feature of a developing foetus is absent.

(D) The correct answer is $D$. The mammalian embryo exhibits a parasitic mode of nutrition because it remains connected to the mother's body via the placenta. Through this connection,it derives all its essential nutrients,oxygen,and metabolic waste removal directly from the maternal blood supply,which is characteristic of a parasitic relationship.

(D) The process of human reproduction follows a specific sequence: Gametogenesis $\rightarrow$ Insemination $\rightarrow$ Fertilization $\rightarrow$ Cleavage $\rightarrow$ Implantation $\rightarrow$ Gestation $\rightarrow$ Parturition.
Implantation refers to the attachment of the blastocyst to the uterine wall.
Once implantation is successful,the period of pregnancy,known as gestation,begins.
Therefore,implantation is followed by the gestation period.

(C) During the process of gastrulation,the blastula is transformed into a gastrula.
This process involves the movement of cells to form the three germ layers (ectoderm,mesoderm,and endoderm).
$A$ new cavity is formed during this stage known as the $Archenteron$ (also called the gastrocoel).
The $Archenteron$ opens to the exterior through an opening called the blastopore.

(A) The $3$ primary germ layers in the human embryo are the ectoderm, mesoderm, and endoderm.
$1$. The $Nervous$ system is derived from the ectoderm.
$2$. The $Muscular$ system, $Circulatory$ system, and $Gonads$ are all derived from the mesoderm.
Therefore, the correct answer is the $Nervous$ system.

(D) During the process of gastrulation in chordates,the archenteron (primitive gut) is formed. The roof of this archenteron is composed of the notochordal plate,which is derived from the chordamesoderm. Therefore,the chordamesoderm forms the roof of the archenteron.

(A) During embryonic development,the three primary germ layers are formed: Ectoderm,Mesoderm,and Endoderm.
$1$. The $Ectoderm$ gives rise to the nervous system,epidermis of the skin,and associated glands.
$2$. The $Mesoderm$ is responsible for the development of the circulatory system (vascular system),excretory system (kidneys),muscular system,skeletal system,and the dermis of the skin.
$3$. The $Endoderm$ forms the epithelial lining of the digestive tract and respiratory system,as well as organs like the liver and pancreas.
Therefore,the vascular system and excretory organs are derived from the $Mesoderm$.

(D) Gastrulation is a critical phase of embryonic development. During this process,the blastula undergoes significant cell movements (morphogenetic movements) to reorganize into a multi-layered structure known as the gastrula. This process results in the formation of the three primary germinal layers: the ectoderm,the mesoderm,and the endoderm. These layers eventually give rise to all the tissues and organs of the adult body.

(A) The $blastopore$ is the opening of the central cavity of an embryo in the early stage of development. It is formed during the process of $gastrulation$,where the single-layered $blastula$ is reorganized into a multilayered structure called the $gastrula$. Therefore,the $blastopore$ is a characteristic feature of the $gastrula$ stage.

(A) The human embryo develops from three primary germ layers: ectoderm,mesoderm,and endoderm.
$1$. The ectoderm gives rise to the epidermis of the skin,the nervous system,and the enamel of the teeth.
$2$. The mesoderm gives rise to the dermis,muscles,bones (including the skull and axial skeleton),and dentine of the teeth.
Therefore,the enamel of the teeth is the only structure listed that is derived from the ectoderm.

(C) During the process of gastrulation,the blastula undergoes rearrangement of cells to form three germ layers.
In this stage,a new cavity is formed which is known as the archenteron or the primitive gut.
This cavity eventually develops into the digestive tract of the embryo.
Therefore,the correct answer is $Archenteron$.

(D) The process of transformation of the blastocyst into the gastrula,involving the migration and rearrangement of cells to form the three primary germ layers ($ectoderm$,$mesoderm$,and $endoderm$),is known as $gastrulation$.