Ch 2Reproduction in Lower and Higher Animals — Class 12 Biology, concept-first.

Reproduction is one of the major life processes of every living organism: it is the biological process by which new individuals arise from pre-existing ones of the same kind, and it is what keeps a sp…

Asexual reproduction is the common mode of reproduction among lower animals. Unlike sexual reproduction, it does not involve meiosis or the formation and fusion of gametes.

A gemmule is a special kind of internal bud formed only in sponges (Phylum Porifera), such as Spongilla, the common freshwater sponge.

Budding is a simple and common method of asexual reproduction, generally occurring when conditions are favourable.

Sexual reproduction (also called amphimixis) is the process by which offspring are produced through the formation and fusion of gametes, and in animals gamete formation primarily involves meiosis.

A. Male Reproductive System : It consists of the primary male organ (gonad), the testes, and the accessory ducts and glands that form the internal and external genitalia:

a. Testes : The testes are a pair of mesodermal organs formed in the lower abdominal cavity and housed in a pouch called the scrotum.

b. Accessory ducts : The accessory ducts include the rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory duct and urethra; all of these except the urethra are present in pairs.

c. Glands : The male accessory glands are as follows:

d. External genitalia : The external genitalia of the male include the penis and the scrotum. The penis is the male copulatory organ — a cylindrical, muscular structure built of three bundles of erect…

B. Female Reproductive System : The female reproductive system consists of the following parts:

1. Ovary : The ovary is the primary female sex organ, responsible for producing the egg (ovum) and the female reproductive hormones.

The Graafian follicle is the mature ovarian follicle, ready to release its egg at ovulation. Inside it, an eccentrically placed secondary oocyte is surrounded by a non-cellular layer called the zona p…

2. Oviduct / Fallopian tube / Uterine tube : These are a pair of muscular ducts lying horizontally over the peritoneal cavity, with their proximal end close to the ovary and their distal end opening i…

3. Uterus : It is commonly called the womb, a a hollow, muscular, pear-shaped organ located above and behind the urinary bladder, about 7.5 cm long, 5 cm broad and 2.5 cm thick.

4. Vagina : It is a tubular female copulatory organ, 7–9 cm in length, lying between the cervix and the vestibule.

5. External genitalia : The external genital organs of the female lie outside the vagina and are collectively called the vulva (meaning 'covering' or 'wrapping') or pudendum.

6. Accessary glands / Vestibular glands / Bartholin's glands : These are a pair of small glands forming part of the female reproductive system alongside the ovaries, oviducts, uterus, vagina and mamma…

Mammary glands : Accessory organs of the female reproductive system, responsible for producing and releasing milk after parturition.

Puberty / Sexual maturity in Males : Puberty is the age at which the reproductive system becomes functional, the sex organs begin producing gametes and sex hormones, and secondary sexual characters ap…

The menstrual cycle, also called the ovarian cycle, is a characteristic feature of primates, including humans.

a. Menstrual phase : The beginning of each cycle is taken as the first day of menses — the loss of blood (about 45–100 ml) — which lasts on average about 3–7 days (roughly 5 days).

b. Proliferative phase / Follicular phase / Post menstrual phase : This is the period between the end of menstruation and ovulation (the release of the ovum); its duration is more variable than the ot…

c. Ovulatory phase : It is the shortest phase of the menstrual cycle. It involves the rupturing of the mature Graafian follicle and the release of the ovum (actually the secondary oocyte) into the pel…

d. Secretory phase / Luteal phase : This phase lasts from ovulation to the beginning of the next menses — it is the longest phase of the cycle, lasting about 14 days, from roughly the 15th to the 28th…

Gametogenesis is the process by which gametes are formed in sexually reproducing animals. The male gamete is the sperm (spermatozoon) and the female gamete is the ovum or egg; both are ultimately deri…

Spermatogenesis is the process by which the male gamete, the spermatozoon, is formed from the germinal epithelium of the testis; it begins at puberty, when the hypothalamus starts secreting gonadotrop…

Structure of sperm : The sperm is the male gamete, a motile, microscopic, elongated cell divisible into three parts: head, middle piece and tail.

Oogenesis : It is the process by which the haploid female gamete — the egg or ovum — is formed from the diploid germinal epithelium of the ovary; like spermatogenesis, it involves meiosis (and, earlie…

Structure of secondary oocyte : In humans, what is popularly called the 'unfertilized egg' — the cell released from the ovary at ovulation — is, strictly, the secondary oocyte.

Sexual reproduction primarily involves the formation and fusion of gametes; fertilization is this later process — the fusion of the haploid male and female gametes, resulting in the formation of a dip…

Mechanism of fertilization : Semen released at ejaculation contains sperm along with various secretions; the coagulated semen soon liquefies, and the sperm become active.

Significance of fertilization :

The zygote formed as a result of syngamy is activated to divide, entering a sequence of developmental stages.

Cleavage : It is the process of early, rapid mitotic division of the zygote, converting it into a hollow, multicellular blastula; unlike ordinary cell division, cleavage does not involve growth of the…

Blastulation : It is the process by which the solid morula transforms into the hollow, multicellular blastocyst.

Implantation : It is the process by which the blastocyst, after its formation, becomes embedded within the endometrium of the uterus.

Gastrulation : It is the process by which the 'gastrula' is formed from the blastocyst. In the gastrula stage the rate of cleavage slows, but two important events take place actively:

Once the three germ layers — ectoderm, mesoderm and endoderm — are established at gastrulation, each is committed to giving rise to a characteristic, non-overlapping set of tissues and organs of the a…

Pregnancy is the condition of carrying one or more embryos within the uterus; it is also called gestation, and refers to the whole period between fertilization of the egg and parturition.

First Trimester : (Period from fertilization to 12th week) It is the period of the most radical changes in both mother and embryo.

Second Trimester : (Period from 13th to 26th week) It is a period of rapid growth of the foetus. The uterus grows enough that the pregnancy becomes visibly obvious.

Third (final) Trimester : (Period from 27th week till the parturition) The foetus grows to about 3–4 kg in weight and 50 cm in length, and its eyes open.

The placenta is a flattened, discoidal organ formed within the uterus of a pregnant woman, providing a temporary structural and functional connection between the foetal and maternal circulations.

Humans are viviparous, giving birth to live young; parturition is this process of giving birth to a baby.

By the process of lactation, the mammary glands of the female begin producing milk towards the end of pregnancy.

According to the World Health Organisation (WHO), reproductive health means total wellbeing in all aspects of reproduction — its emotional, behavioural and social aspects, along with the purely physic…

Birth-control measures that deliberately prevent fertilization are called contraceptives, and contraceptive methods exist to help prevent unwanted pregnancies.

a. Temporary methods : These are of the following types:

b. Permanent Method : The permanent birth-control methods are vasectomy in men and tubectomy in women; both are surgical methods, also called sterilisation, and unlike temporary methods they are not i…

Medical Termination of Pregnancy (MTP) : An intentional or voluntary termination of pregnancy before full term is called MTP or induced abortion; it is the intentional or voluntary termination of a pr…

Diseases or infections transmitted through sexual intercourse are collectively called sexually transmitted diseases (STDs), venereal diseases (VDs), or reproductive tract infections (RTIs).

Infertility is defined as the inability to conceive naturally after (typically) one year of regular, unprotected intercourse.

Prevention therefore starts with the lifestyle causes that are within a couple's control; the physical and congenital causes listed above need medical treatment or one of the assisted reproductive tec…

Before 1978, an infertile couple had two options: adopt, or remain childless. Today infertile couples have many options to have a child, such as fertility drugs, test-tube babies, artificial inseminat…

Q. 1 of the exercise (printed page 47): eight multiple-choice questions on the zygote, the primary sex organ, implantation, ovulation, the timing of implantation, the test-tube-baby technique, the par…

Q. 2 of the exercise (printed page 47): eight one-sentence questions, from the difference between a foetus and an embryo to the differences between blastula and gastrula, answered in the one-line form…

Q. 3 of the exercise (printed page 47): six fill-in-the-blank sentences on the primary sex organ, the womb, the site of fertilization, the placenta, the gonorrhoea bacterium and the testis hormone.

Q. 4 of the exercise (printed page 48): seven short-answer questions on budding in Hydra, reproduction in sponges, IVF, mechanical contraceptives, tubectomy, syphilis and colostrum, each written to th…

Q. 5 of the exercise (printed page 48): ten descriptive questions, the menstrual cycle and its hormones, parturition, the histology of testis and ovary, the blastula, birth control, the RCH programme,…

Q. 6 of the exercise (printed page 48): four long-answer questions, the male reproductive system with a labelled diagram, the female reproductive system, fertilization, and cleavage of the zygote to t…

The chapter closes (printed page 48) with a project: a chart of other sexually transmitted diseases with their causal organisms, symptoms and control measures.

Sample papers and previous-year board questions for this subject.