Maharashtra Board Class 12 Biology Chapter 2 Reproduction in Lower and Higher Animals PDF Download

Reproduction in Lower and Higher Animals

Gemmule Formation

Gemmule is an internal bud formed only in sponges. It has asexually produced mass or aggregation of dormant cells, the archaeocytes capable of developing into a new organism. The archaeocytes get coated by a thick resistant layer of secretion by amoebocytes. The gemmule is formed to overcome unfavourable conditions. On return of favourable conditions of water and temperature, the gemmules hatch and develop into a new individual. e.g. Spongilla.

We know that reproduction is one of the major life processes of any living organism. It helps in maintaining the continuity of the species. Reproduction is defined as the biological process of formation of new life forms from pre-existing similar life. It thus becomes a vital process which enables the species to survive over a long period, even though the individuals or organisms live naturally for a limited period of time i.e. their life span. In this chapter, we will learn about the various methods of reproduction in animals the human reproductive system, gametogenesis and fertilization, early embryology, parturition and reproductive health.

Reproduction in animals occurs mainly by two methods i.e. asexual and sexual.

2.1 Asexual Reproduction in Animals

It is a common method among lower animals. It does not involve meiosis nor the gamete formation and fusion. The formation of progeny is by a single parent only and does not involve both the sexes, so it is called asexual reproduction. The progeny or daughter cells are genetically identical to the single parent and are also referred to as clones. The lower animals reproduce asexually by gemmule formation and budding.

Budding

It is a simple method of asexual reproduction normally occuring in favourable conditions. It is seen in a variety of animals like coelenterates (Hydra and corals) and in some colonial ascidians. In Hydra, a small outgrowth is produced towards the basal end of the body.

Teacher's Note

Budding is like making a small copy of a parent animal. Just like a branch grows from a tree, a baby Hydra grows from the parent Hydra.

Exam Trick

Remember: Asexual reproduction = only one parent. Sexual reproduction = two parents. Budding in Hydra is asexual because only one parent makes babies.

Points to Remember

Gemmule is formed only in sponges.
Budding happens in Hydra and corals.
Asexual reproduction needs only one parent.
Clones are exact copies of the parent.
Gemmule helps sponges survive bad weather.

2.2 Sexual Reproduction in Animals

It is the process which involves the production of offspring by the formation and fusion of gametes. It is also called amphimixis. In animals, gamete formation primarily involves meiosis.

The sexually reproducing animals show two main phases in their life time. The earlier juvenile phase mainly represents physical growth phase starting from birth. The animals can not reproduce sexually in this phase. The later Reproductive maturity phase is attained usually after physical growth is almost over. It involves growth and activity of the sex organs. The animal can reproduce sexually in this phase. Both these periods (phases) are of variable duration in different animals. After attaining sexual maturity, the animal exhibits various events, namely pre-fertilization (gametogenesis and gamete transfer), fertilization (fusion of male and female gametes) and post fertilization events (formation of zygote and embryogenesis).

The sexually reproducing animals show various breeding patterns. Some like the goat, sheep, and donkey are seasonal breeders while humans and apes are continuous breeders. They can breed throughout the year.

Human Reproduction

Humans are sexually reproducing animals. The process of reproduction involves various sequential steps such as gametogenesis, insemination, internal fertilization (i.e. fusion of male and female gametes), zygote formation and embryogenesis, gestation and parturition.

The gametes, sperms and eggs are produced by the primary sex organs, testis in male and ovary in female. Organs other than testis and ovary, are called secondary sex organs of the male and female. As male and female can be externally differentiated by certain specific features called secondary sexual characters, they are called sexual dimorphic characters. In males, presence of beard, moustache, hair on the chest, muscular body, enlarged larynx (Adam's apple) are secondary sexual characters while in females these characters are the developed breast, broader pelvis and high pitched voice.

Teacher's Note

Sexual reproduction needs two parents - a mother and a father. In humans, the baby grows inside the mother's body before birth.

Exam Trick

Remember: Sexual = two (male and female). Asexual = one (only parent). Just think: sexual needs "sex" which means two different parents!

Points to Remember

Sexual reproduction needs a male and female parent.
Gametes are sex cells (sperm and egg).
Meiosis makes gametes with half the chromosomes.
Humans are continuous breeders (can breed all year).
Secondary sexual characters show if someone is male or female.

A. Male Reproductive System

It consists of the primary male organ (gonad) called testes, the accessory ducts and glands which form internal and external genitalia.

a. Testes

A pair of testes, mesodermal in origin, are formed in the lower abdominal cavity. They are located in a pouch called scrotum. During early foetal life, the testes develop in abdominal cavity and later they descend into the scrotal sac through a passage called inguinal canal. Each testis is oval in shape, 4 to 5cm long, 2 to 3cm wide and 3cm thick.

Histology of Testis

The testis is externally covered by a collagenous connective tissue layer called tunica albuginea. Outer to it is an incomplete peritoneal covering called tunica vaginalis, and inner to it is tunica vasculosa, a thin membranous and vascular layer. Fibers from tunica albuginea divide each testis into about 200-300 testicular lobules (refer dig. 2.3 L. S. of testis). Each with 1-4 highly coiled seminiferous tubules. Each seminiferous tubule is internally lined by cuboidal germinal epithelial cells (spermatogonia) and few large pyramidal cells called Sertoli or sustentacular cells.

The germinal epithelial cells undergo gametogenesis to form the spermatozoa. Sertoli cells provide nutrition to the developing sperms. Various stages of spermatogenesis can be seen in the seminiferous tubules. The inner most spermatogonial cell (2n), primary spermatocyte (2n), secondary spermatocyte (n), spermatids (n) and sperms (n). The Interstitial or Leydig's cells lie in between the seminiferous tubules. They secrete the male hormone androgen or testosterone.

Teacher's Note

Testes are the male sex organs that make sperm. Think of them like a factory that produces baby-making cells called sperm.

Exam Trick

Remember: Testis makes sperm. Ovary makes eggs. Testosterone is made by Leydig cells in testis.

Points to Remember

Testes are located in the scrotum (a pouch).
Seminiferous tubules make sperm inside the testis.
Sertoli cells feed and support growing sperm.
Leydig cells make testosterone hormone.
Each testis has 200-300 tiny lobules (sections).

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