NCERT Solutions Class 12 Biology Chapter 3

NCERT Solutions Class 12 Biology Chapter 3: Human Reproduction

NCERT Solutions Class 12 Biology Chapter 3 is about reproduction in humans. Reproduction is a biological practice in which an organism gives rise to young ones similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death. 

Reproduction facilitates the continuation of the species, generation after generation, which you can study in NCERT Solutions Class 12 Biology Chapter 3. 

This chapter deals with the study of sexual reproduction consisting of a set of events and can be divided into three stages: 

  • Pre-fertilization
  • Fertilization
  • Post-fertilization. 

By referring to Extramarks’s NCERT Solutions Class 12 Biology Chapter 3, students will gain good knowledge and write precise answers in the examination. These notes have been designed by teachers with several years of experience in teaching this subject. 

Extramarks’ objective is to assist students of Class 12 by offering all the knowledge about the examinations, important questions, and the CBSE syllabus, including NCERT Solutions Class 12 Biology Chapter 3. The notes prepared by the Extramarks will give you an understanding of the critical concepts and smoothen your learning process.

Key Topics Covered In NCERT Solutions Class 12 Biology Chapter 3

Basic Steps in Human Reproduction:

  1. Gametogenesis.
  2. Insemination.
  3. Fertilization.
  4. Blastocyst development.
  5. Implantation.
  6. Embryo development.
  7. Parturition.

1.Male Reproductive System:

There are four primary parts of the male reproductive system:

  1. Testes.
  2. Accessory ducts.
  3. Glands
  4. External Genitalia.

Testes (Singular Testis):

  • The scrotum is located in the pelvic region outside the abdominal cavity within a pouch.
  • A tiny muscular sac is called a Scrotum that contains and protects the testes. The scrotum is situated behind the penis and is treated as a part of the outer male genitalia.
  • The testes are positioned outside the abdominal cavity to defend the moderate temperature of the testes, nearly 2-2.5◦C, lesser than the average human body temperature. This condition is essential for the synthesis of sperms.
  • The shallow temperature in testes is necessary for spermatogenesis as the standard human body temperature can lead to mutation in the sperm.
  • The testis is oval with a length of 4-5cm and 2-3cm wide.
  • The testis consists of about 250 compartments called Testicular lobules.
  • Each lobule generally constitutes 1-3 highly coiled seminiferous tubules that play a crucial role in sperm production.
  • Seminiferous tubules are the place that creates spermatozoa by the method of meiosis.
  • Each seminiferous tubule has two types of cells, i.e., Sertoli cells and spermatogonia in their inner lining.
  • These spermatogonia are diploid in the environment and called the immature germ cells, and they shape the sperm by the procedure of meiosis. They hold 46 chromosomes in their cells.
  • Sertoli cells in seminiferous tubules offer nutrition to the spermatogonia.
  • Interstitial spaces are the region outside the seminiferous tubule that carries the narrow Leydig cells, blood vessels, and some immunocompetent cells.
  • Leydig cells synthesize and emit the testicular hormones known as androgens.

Accessory Ducts:

The accessory ducts facilitate the movement of the sperm from the testes to the urethra for their discharge outside the physique. The male reproductive system includes four accessory ducts:

  • Rete Testis

These are the duct points where the seminiferous tubules start into a sequence of channels.

  • Vasa Efferentia: 

These ducts make a pathway to shift the sperm from the rete testis to the Epididymis, which is located on the posterior surface of every testis.

Epididymis

It is a long, coiled tube that joins a testicle to a vas deferens. The Epididymis is placed on the backside of every testicle.

Vas Deferens: 

Vas deferens is a muscular duct that mounts into the abdominal cavity and constructs a loop above the urinary bladder. It transmits the sperm from the Epididymis to the ejaculatory duct.

Ejaculatory Duct: 

This duct is placed on each side of the prostate gland. Ejaculatory ducts store and supply the sperm from the testis to the exterior via the urethra.

Urethra

The urethra is a thin muscular tube that derives from the urinary bladder and then passes the penis to its external opening called the urethral meatus.

Accessory Glands:

There are primarily three kinds of accessory glands. These glands secrete the seminal plasma that principally contains calcium, fructose, and certain enzymes. This secretion blends with the sperms to nourish and guard them.

Seminal Vesicles

The Seminal vesicles contribute an essential proportion of approximately 60-75% of the fluid in semen. This fluid is high in proteins, enzymes, vitamin C, prostaglandins, fructose, and phosphorylcholine. 

Prostate Gland: 

The prostate gland is a thick composition positioned just menial to the urinary bladder. The discharge of the prostate gland is slightly thin, alkaline, and milky-coloured. It helps in the stability of sperm in the acidic vaginal condition and also develops the motility of the sperm.

Bulbourethral Glands

The secretion of these glands assists in the lubrication of the penis and also neutralizes any residual acidity in the urethra. 

  1. Female Reproductive System:

The human female reproducing system is specialized to carry out various functions like ovulation, gametogenesis, fertilization, birth, pregnancy, and child care. The female reproductive structure is made up of numerous parts:

  1. Ovaries:
  • The ovaries are mini and oval-shaped and are known as critical female sex organs.
  • They generate the female gamete, an ovum, and produce ovarian hormones.
  • The ovaries are about 2-4cm in length.
  • They are linked to the pelvic wall and uterus with the assistance of ligaments.
  • The ovaries are secured by a thin epithelium that encloses the ovarian stroma.

Ovarian stroma is the matrix of the ovary that is split into double regions:

  1. Peripheral cortex.
  2. Inner medulla.

Accessory Ducts

The accessory ducts of the female reproducing system consist of the oviducts, uterus, and vagina.  

Oviduct (Fallopian Tube):

  • A fallopian tube usually is 10-12 cm in length.
  • It shields from the periphery region of every ovary to the uterus.
  • Infundibulum: It is a proximal element of the oviduct near to the ovary. It is funnel-shaped and possesses fimbriae.
  • Fimbriae: They are the finger-like projections at the infundibulum’s edges. They assist in the compilation of the ovum after ovulation.
  • Ampulla: This is the broad part of the oviduct that fixes with the infundibulum.
  • Isthmus: This is the last segment of the oviduct that expands through the uterine walls and opens into a small lumen.

Uterus:

  • The uterus has also been labelled as the womb.
  • Ligaments are crafted like an inverted pear and attached to the pelvic wall.
  • In the uterus, the embryo grows into the fetus.
  • It opens into the vagina via a slim cervix.
  • The uterine wall has three layers of tissues: 
  • Perimetrium.
  • Myometrium.
  • Endometrium.

Cervix:

  • It is a thin canal, usually 2 to 3 cm in length, that attaches the uterus to the vagina.
  • Cervical Canal: The cervical canal bridges the cavity of the physique of the uterus with the lumen of the vaginae.
  • Birth Canal: The cervical canal across the vagina shapes the birth canal, which aids during the baby’s delivery.

External Female Genitalia:

The primary external female genitalia of the female reproductive system is made up of the mons pubis, labia minora, labia majora, hymen, and clitoris.

Hymen

The hymen is a thin membrane that revolves around the opening of the vagina. It is often torn during the first coitus or intercourse, or it can also be broken by participation in some sports like cycling, horseback riding etc.

Clitoris

It is a sensitive, small, finger-like projection at the junction of the labia minora over the urethral opening. 

Mammary Glands:

All-female mammals own a couple of functional mammary glands. The breasts are a paired system of glandular tissue and a variable amount of fats. Each mammary gland has 15-20 mammary lobes.

  1. Gametogenesis:

It is the system by which the fundamental male and female sex organs like the testes in males and the ovaries in the female form gametes.

Spermatogenesis:

This is the technique by which the immature male germ cells, also called spermatogonia, bring out mature sperm cells in the testis.

Oogenesis: 

Oogenesis is the practice by which the immature oogonia in the ovaries generates a mature ovum.

Spermatogenesis

The spermatogenesis process starts at puberty.

Spermatogonia:

They are the incomplete male germ cells that experience meiotic divisions to configure sperm. Spermatogonium (singular) is diploid in mother nature and contains 46 chromosomes.

Primary Spermatocytes: 

These are the spermatogonia that live through the meiosis procedure to create two equal haploid cells called secondary spermatocytes. They accommodate 46 chromosomes in every cell.

Secondary Spermatocytes: 

They are haploid and derive from the main spermatocytes due to meiosis . They encompass 23 chromosomes.

Spermatids: 

These cells arise from the subordinate spermatocytes as an action of meiosis II. Spermatids are haploid cells that incorporate 23 chromosomes.

Spermiogenesis: 

It is the method by which spermatids mature to create spermatozoa or sperms.

Spermiation: 

This is the system by which sperms are delivered from the seminiferous tubules. 

  1. Menstrual Cycle:

This is the reproductive pattern that initiates from one menstruation to the next. It primarily takes place in female primates like apes, monkeys, and human beings. The rhythm repeats at 28-35 days intervals and usually releases one egg per cycle. This rotation is essential for the production of oocytes and the devising of the uterus for pregnancy. 

Menstruation: 

In this routine, the blood and mucosal tissue are discharged periodically. It happens due to the breakage of the inner lining of the uterus. This mechanism occurs once a month and is known as period.

Menarche: 

Menarche is the initial menstruation of a human female that starts at puberty. 

Menopause

Menopause is explained as the permanent end of the menstrual cycle in females.

 Phases of the Menstrual Cycle:

The menstrual cycle follows four phases:

  1. Menstrual phase.
  2. Follicular phase.
  3. Ovulation.
  4. Luteal Phase.
  5. Fertilization and Implantation:

Insemination: 

It is how semen is released into the vagina by the penis during coitus or copulation.

Fertilization:

It mixes the haploid male gamete or sperm and the haploid female egg.

Morula

The embryo with 8-16 blastomeres is referred to as a morula. It continues its division as it transfers further along into the uterus. The blastomeres are assembled into double layers. The outer layer is termed the trophoblast, and the other is the inner cell mass which is attached to the trophoblast.

NCERT Solutions Class 12 Biology Chapter 3: Exercises and Answer Solutions

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Key Features of NCERT Solutions Class 12 Biology Chapter 3

Some of the primary key features of the NCERT Solutions Class 12 Biology Chapter 3 are as under:

  • NCERT Solutions Class 12 Biology Chapter 3 will guide students in formulating their answers effectively in the examination. 
  • The theory described in these notes is detailed so students can compose crisp and clear Solutions based on the content available on the Extramarks.
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Q.1 Fill in the blanks:
(a) Human reproduce . (asexually/sexually)
(b) Humans are . (oviparous/viviparous/ovoviviparous)
(c) Fertilisation is in humans. (external/internal)
(d) Male and female gametes are . (diploid/haploid)
(e) Zygote is (diploid/haploid)
(f) The process of release of ovum from a mature follicle is called .
(g) Ovulation is induced by a hormone called .
(h) The fusion of male and female gametes is called .
(i) Fertilization takes place in .
(j) Zygote divides to form which is implanted in uterus.
(k) The structure which provides vascular connection between foetus and uterus is called .

Ans-

(a) sexually

(b) viviparous

(c) internal

(d) haploid

(e) diploid

(f) ovulation

(g) luteininsing hormone

(h) fertilisation

(i) fallopian tube

(j) blastocyst

(k) placenta (umbilical cord)

Q.2 Draw a labelled diagram of male reproductive system.

Ans-

Q.3 Draw a labelled diagram of female reproductive system.

Ans-

Q.4 Write two major functions each of testis and ovary.

Ans-

Two major functions of testis:

(a) Production of sperms in the highly coiled seminiferous tubules

(b) Secretion of a male hormone called testosterone by interstitial cells

Two major functions of ovary:

(a) Production of female gamete (ovum) every month by the process of oogenesis

(b) Production and secretion of female sex hormones, estrogen and progesterone, during the ovarian cycle

Q.5 Describe the structure of a seminiferous tubule.

Ans-

Seminiferous tubules are highly coiled structures found in each lobule of testes and are 1-3 in numbers. The combined length of all the tubules found in the testes is approximately 250 m. Each seminiferous tubule is lined on its inside by two types of cells namely

  • Male germ cells or spermatogonia
  • Sertoli cells

It is packed with cells undergoing spermatogenesis. The Sertoli cells not just provide the support and nourishment (nutrients) to the sperms, but also regulate the spermatogenic cells. The regions outside the seminiferous tubules are called interstitial spaces. They contain small blood vessels and interstitial or Leydig cells. Leydig cells are polyhedral in shape. They synthesise and secrete testicular hormones called androgens.

Diagrammatic sectional view of seminiferous tubule

Q.6 What is spermatogenesis? Briefly describe the process of spermatogenesis.

Ans-

The process of production of male gametes (sperms) in the primary male sex organ testes is called spermatogenesis. This process starts at the onset of puberty, where immature male germ cells or spermatogonia are converted into sperms. The diploid spermatogonia which are present on the inside wall of seminiferous tubules undergo mitotic division and increase their number. Out of these, some are called primary spermatocytes and they undergo meiotic division periodically. A primary spermatocyte completes the first meiotic division leading to the formation of two equal, haploid cells called secondary spermatocytes (having only 23 chromosomes). The secondary spermatocytes undergo the second meiotic division and thus produce four equal, haploid spermatids. Spermatids also have 23 chromosomes and are thus; haploid. The process of spermiogenesis further transforms these spermatids into spermatozoa (sperms) after which the sperm head gets embedded in the Sertoli cells. As the last step, they are released from the seminiferous tubules by the process called spermiation.

Q.7 Name the hormones involved in regulation of spermatogenesis.

Ans-

The hormones involved in regulation of spermatogenesis are gonadotropin releasing hormone (GnRH), luteinizing hormone (LH), follicle stimulating hormone (FSH) and androgens (testosterone).

Functions of Hormones:

  1. Spermatogenesis starts at the onset of puberty due to a significant increase in the secretion of gonadotropin releasing hormone (GnRH), which is released from the hypothalamus.
  2. The increased level of GnRH results in the secretion of gonadotropin hormone namely luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH acts on Leydig cells and stimulates synthesis and secretion of androgens (testosterone).
  3. Androgens stimulate the process of spermatogenesis.
  4. FSH acts on the Sertoli cells and stimulates the secretion of factors that are also involved in the process of spermiogenesis.

Q.8 Define spermiogenesis and spermiation.

Ans-

  1. Spermiogenesis: The final stage of spermatogenesis in which haploid spermatids, produced as a result of meiosis from spermatocytes, are converted into mature and motile spermatozoa or sperm is called spermiogenesis.
  2. Spermiation: The process by which embedded sperms, produced as a result of spermiogenesis, are released from the Sertoli cells into the lumen of seminiferous tubules is called spermiation.

Q.9 Draw a labelled diagram of sperm.

Ans-

Q.10 What are the major components of seminal plasma?

Ans-

Seminal plasma is a mixture of secretions from three different glands (seminal vesicles, prostate and paired bulbourethral glands) of male reproductive systems. Prostate gland secretes a milky white alkaline fluid which enhances the sperm motility. The secretion from bulbourethral gland is mucoid in nature and has lubricating effect. The combination of secretion from these three different glands constitutes seminal plasma. This plasma is rich in nutrients like fructose, calcium and certain enzymes and is a thick, viscous fluid.

Q.11 What are the major functions of male accessory ducts and glands?

Ans-

The main accessory ducts of the male reproductive system are rete testis, vasa efferentia, epididymis and vas deferens. Their major function is storage and transportation of sperms from the testis to the outside through urethra.

The male accessory glands are seminal vesicles, prostate and paired bulbourethral glands. Their combined secretion is called seminal plasma which is rich in fructose, calcium and certain enzymes. They provide nutrients to the sperm and also serve as a lubricant of the penis.

Q.12 What is oogenesis? Give a brief account of oogenesis.

Ans-

The process of formation of a mature female gamete is called oogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells or oogonia are formed in each fetal ovary. These cells start dividing and enter prophase-I of the meiotic division and get temporarily arrested at that stage. They are called primary oocytes at this stage. The primary oocytes then get surrounded by a layer of granulosa cells and this structure is called primary follicle. Many of the primary follicles degenerate from birth to puberty phase. About 60,000-80,000 such primary follicles remain at the onset of puberty. The primary follicle gets surrounded by more layers of granulosa cells and a new theca. This structure is then called secondary follicle. Out of thousands of such follicles, only about 400 ever mature because a female produces only one egg per month during her reproductive years.

The secondary follicle soon transforms into a tertiary follicle with a fluid-filled cavity called antrum with the further organisation of theca layer. It is during this stage that the primary oocyte grows in size and completes its first meiotic division. It is an unequal division that results in the formation of a large haploid secondary oocyte and a tiny polar body. The secondary oocyte retains most of the nutrient-rich cytoplasm. This tertiary follicle changes into a mature Graafian follicle. The secondary oocyte forms a new membrane, called zona pellucida, surrounding it. The Graafian follicle ruptures and the secondary oocyte or ovum is released from the ovary. This process is called ovulation.

The ovarian cycle is under the control of gonadotropic hormones, follicle-stimulating hormone (FSH) and luteinising hormone (LH). It is the interplay between these two hormones that control the ovulation cycle.

Q. Draw a labelled diagram of a section through ovary.

Ans-

Q.14 Draw a labelled diagram of a Graafian follicle.

Ans-

Q.15 Name the functions of the following:
(a) Corpus luteum
(b) Endometrium
(c) Acrosome
(d) Sperm tail
(e) Fimbriae

Ans-

(a) Corpus luteum: Corpus luteum is the transformed and ruptured Graafian follicle after ovulation phase. During the luteal phase of the menstrual cycle, corpus luteum secretes large amounts of progesterone which is essential for the maintenance of the endometrium and making it ready for implantation of the fertilised ovum and other events of pregnancy

(b) Endometrium: The endometrium functions as the inner lining of the uterus, preventing adhesions between the opposed walls of the myometrium. During the menstrual cycle, the endometrium grows to a thick, blood vessel-rich, glandular tissue layer. This provides optimal environment for the implantation of a blastocyst upon its arrival in the uterus. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the developing embryo.

(c) Acrosome: Acrosome is a cap-like structure found at the head of a sperm that comes in contact with the ovum at the time of fertilisation and releases a hydrolytic enzyme, called hyaluronidase, that hydrolyses the outer membrane of the ovum and thus, helps in the process of fertilisation.

(d) Sperm tail: The sperm tail is made of protein fibres that contracts on the alternative sides, performing a wave-like movement. This imparts motility to the sperm and facilitates the movement of the sperm inside the female reproductive tract so that it can reach the ovum to fertilise it .

(e) Fimbriae: Fimbriae are finger-like projections found at the edges of infundibulum, which is the ovarian end of the fallopian tube). This structure helps in the collection of ovum after ovulation with the help of ciliary movement

Q.16 Identify True/False statements. Correct each false statement to make it true.

  1. Androgens are produced by Sertoli cells. (True/False)
  2. Spermatozoa get nutrition from Sertoli cells. (True/False)
  3. Leydig cells are found in ovary. (True/False)
  4. Leydig cells synthesise androgens. (True/False)
  5. Oogenesis takes place in corpus luteum. (True/False)
  6. Menstrual cycle ceases during pregnancy. (True/False)
  7. Presence or absence of hymen is not a reliable indicator of virginity or sexual experience. (True/False)

Answer

  1. False, androgens are not produced by Sertoli cells. Sertoli cells are found inside the seminiferous tubules. Androgen is produced by interstitial cells or Leydig cells found in the regions outside the seminiferous tubules.
  2. True
  3. False. Leydig cells are not found in ovary. Leydig cells are found in the regions outside the seminiferous tubules, called interstitial spaces.
  4. True
  5. False, oogenesis does not take place in corpus luteum. Oogenesis takes place in Graafian follicle found in the ovary.
  6. True
  7. True

Q.17 What is menstrual cycle? Which hormones regulate menstrual cycle?

Ans-

A series of cyclic physiological changes that occur inside the female reproductive tract of the primates from the beginning of one menstrual period to the beginning of the next is called the menstrual cycle. In humans, this cycle consists of about 28/29 days. One ovum is released during the middle of each cycle and the endometrium of the uterus also undergoes a cyclic series of events. The major events of the menstrual cycle are as follows:

(a) Menstrual phase (Days 1-5): During days 1-5, there is a low level of female sex hormones in the body, causing the uterine lining to disintegrate and its blood vessels to rupture. A flow of blood, known as menses, passes out of the vagina during a period of menstruation.

(b) Follicular Phase (Day 6-13): There is increased production of estrogen by growing ovarian follicle which causes endometrium of uterus to thicken, and become vascular and granular. The primary follicle in the ovary grows to become a fully mature Graafian follicle. This is called the proliferative phase of the menstrual cycle. These changes in the ovary and uterus are induced by changes in the levels of pituitary and ovarian hormones. Gonadotrophins like luteinising hormone (LH) and follicle-stimulating hormone (FSH) increase gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles. LH and FSH attain their peak level in the middle of the cycle (14th day). The maximum level of LH during the mid-cycle induces rupture of Graafian follicle and results in ovulation at the end of follicular or proliferative phase.

(c) Ovulatory Phase (Day 15-28): This Graafian follicle after rupture turns into corpus luteum. There is increased production of progesterone hormone from the developing corpus luteum. This results in a doubling of endometrium thickness and the uterine glands mature. A thick mucoid secretion is produced and is called secretory phase of the cycle. The endometrium is ready to implantation of a fertilised ovum however in case pregnancy does not occur, the corpus luteum degenerates and the uterine lining breaks down due to low levels of sex hormones. This results in menstruation. While menstruation is going on, the anterior pituitary begins to increase its production of FSH and a new follicle begins to mature.

Cyclic menstruation is an indicator of normal reproductive phase and extends between menarche (onset of menstruation) and menopause (menstruation stops). Gonadotrophins like Luteinizing hormone (LH), follicle stimulating hormone (FSH) and ovarian hormones like progesterone and estrogen regulate the menstrual cycle.

Q.18 What is parturition? Which hormones are involved in induction of parturition?

Ans-

Parturition is the process of delivery of the foetus (childbirth) due to vigorous contractions of the uterus at the end of nine months of pregnancy. This is induced by a complex neuroendocrine mechanism. The signals for parturition are generated from the fully developed foetus and placenta which generates mild uterine contractions. This triggers the release of hormone oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscles and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin. The other hormone that is involved in parturition is relaxin which is responsible for relaxation of ligaments of the pelvic region and thus; helps in childbirth.

Q.19 In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?

Ans-

In our society, women are often blamed for giving birth to daughters. However, this is a completely wrong statement as the sex of the child depends upon the sex chromosome of the sperm that has fused with the mother’s egg giving rise to the zygote. All human beings have 23 pairs of chromosomes out of which, 22 pairs are autosomes and 23rd pair is the sex chromosome. Females have 22 pairs of autosomes and XX chromosomes as their sex chromosome. In the case of males, 22 pairs are autosomes and there are two different kinds of sex chromosomes namely X and Y. When gametes (sperms) are formed in males, some receive X sex chromosome, while others receive Y sex chromosome. Now, depending upon the sperm type (carrying either X or Y chromosome) that fuses with the ovum, the sex of the foetus is determined. Fertilisation with an X chromosome carrying sperm will result in a female child, while that with Y carrying sperm will result in a male child. Thus, it is incorrect to blame women for the gender of the child.

Q.20 How many eggs are released by a human ovary in a month? How many eggs do you think would have been released if the mother gave birth to identical twins? Would your answer change if the twins born were fraternal?

Ans-

A single egg is released by human ovary every month by the process of ovulation. Only a single egg is released in the ovary when the mother gives birth to identical twins. The blastomeres separate during early zygotic stage giving rise to identical twins.

When the twins born are fraternal, two eggs are released in the ovary during a single ovulation event and both are fertilised by two separate sperms leading to the formation of two zygotes.

Q.21 How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?

Ans-

Dogs are polyovulatory non-seasonal species. More than one egg is released from the ovary in each ovulation cycle. Thus, if a female dog gave birth to 6 puppies, it means that 6 eggs were released during ovulation and they all got fertilised.

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