All organisms (Plants and Animals) are not immortal. Life is short even for those who live longest. Growth soon leads to maturity, maturity to old age, old age to death. But before death overtakes it, the organism can beget other creatures like itself. Puppies are born and grow into dogs ; Kittens born and grow into cats ; the race continues, though the individuals die. While all life processes like nutrition, excretion, respiration, respohsiveness etc. are important for the survival of the individual, but reproduction is also essential for the survival of the species. To achieve this goal the organisms have evolved various methods. In this lesson you will learn about the various patterns of reproduction among animals.

• define "reproduction".
• identify growth, maturation, environmental conditions as important factors that influence reproduction.
• list the differences between the two primary types of reproduction i.e. asexual and sexual.
• recognise that sexuality evolved later through bisexuality or hermaphroditism.
• name and describe the different types of asexual reproduction.
• tabulate the different types of fertilization and development with suitable examples.
• recall that in most higher animals there are external sex differences.
• draw labelled diagrams to illustrate sexual reproduction.

1. Mark true or false by writing T/F :

(iv) Internal fertilization always, leads to internal development ( )

(v) Birds and lizards are oviparous animals ( )

2. Give reasons for the following statements:

(i) Sexually produced organisms are better equipped for survival.

(ii) In case of external development, a large number of eggs are required to be laid.

3. Fill in the blanks by selecting suitable alternatives given within brackets.

(i) Two examples of hermaphrodite animals are ........... .......................

and ......................................(Amoeba, Hydra, cockroach, snake,

earthworm).

(ii) Gametes form a ............................... by a process of ..................

(sperm, zygote, embryo, larva, fusion, fragmentation).

Animals may reproduce by two methods :

(a) Asexual reproduction

(b) Sexual reproduction

(a) Asexual - It may be vegetative or by special reproductive units.

(b) Sexual - Fusion of male and female gametes take place

Means of reproduction without the involvement of gametes (ova, sperm). It is of two types:

1. Vegetative reproduction
2. Special reproductive units

(i) Vegetative Reproduction - In vegetative reproduction, 'there is no specialized part of the body which acts as the reproductive unit but the whole of the body of an organism has the capacity to produce a new individual, if broken into two or more parts.

In case of vegatative reproduction, the parent body may break or split into smallest parts or fragments (Fragmentation). This may occur either by itself (autonomously) or as a result of an injury. In either case, each part grows into a complete individual. But when this happens due to an injury it is called Regenerative Reproduction

Regenerative Reproduction is found only in lower animals i.e. some invertebrates.

1. In star fish, a cut arm regenerates all the missing parts to become a whole star-fish. (Fig. 26.1)

(b) Planaria (a flat worm.) if cut into two halves, each part regrows to form a whole individual. (Fig. 26.2). Sponges and Hydra also have very high power of regenerative or vegetative reproduction.

(ii) Special reproductive units - Here reproduction occurs through reproductive units that develop directly to form a new individual. This may occur either by fission or budding.

(a) By Fission : This is typical of unicellular animals like Amoeba (Fig. 26.3), Paramecium.

Takes place by cell division that may be binary (forming two cells) or multiple (forming many cells)

• The entire cell becomes the reproductive unit.
• In case of multiple fission, each daughter cell is called a spore.(fig.26.4)

(b) By budding : Commonly found in lower multicellular animals like Hydra, Tapeworm, etc.

• A few-celled reproductive unit grows out as a projection (bud; from any part of the Parent body. Initially it grows at the cost of the parent body, but later it detaches and grows to fullsize (Fig. 26.5)

• Invertebrates may reproduce both sexually as well as asexually but higher animals i.e. the vertebrates reproduce only sexually,
• "Sexuality" arose as an adaptation.
• Hermaphroditism. Before true sexual form appeared there were organisms that had both sexes within the same body. e.g. Hydra, earthworm, tapeworm.
• Sexual Dimorphism. All vertebrates and some invertebrates-like the arthropods show sexual dimorphism i.e. the two sexes are structurlly differentiated as male and female individuals. (Recall differences between male and female mosquito, frog, lion, peacock or humans, etc.
• Formation of Gametes : The gametes are formed by the process of gametogenesis. This involves meiotic division. Spermatogenesis in testis of male produces the sperms and oogenesis in the ovary of the female produces the eggs cells. Both are haploid i.e. contain half the number of chromosomes that are present in the parent
• Fertilization: The gametes unite by a process called fertilization, this is possible by the act of mating or copulation. During this process the sperms are shed either near the egg cells laid by the female or inside the reproductive tract of the female.

External and Internal Fertilizations : If the fertilization is external, development is also external. Thus, a very large number of egg cells and sperm cells are shed for the possibility of fertilization. This is because, many eggs get lost, or destroyed or eaten by others, e.g., frog. In case the fertilization is internal, the egg cells are protected and hence so many are not required. Development may be external (subsequent to egg laying as in birds snakes, etc.) or intemal(as in dog, cow, humans, etc.).

The young ones : Number of young ones bom at a time, directly depends on the kind of development. More are produced in case of external development,- because many of them do not survive due to hazards of environment, (e.g. frog). Less are produced in case of internal development, as chances of survival are nearly complete and assured (as in mammals).

• Earthworms copulate with their heads in opposite direction by their ventral surfaces (fig.26.6). Each earthworm releases sperms through apertures on 18th segment into sperm receiving sacs located in 5th, 6th, 7th, and 8th segments of the other worm and then they separate.
• Clitellum is a girdle-like structure in 14, 15 and 16 segments, that receives egg under it from the female opening on 14th segment.
• This structure separates from the skin and glides forward towards the head end in each worm. On its way out it collects the sperms from the sperm sacs.
• The clitellum finally slips off the head and both its ends get sealed off, forming a closed cocoon.
• Fertilization takes place inside this cocoon.
• Young ones hatch out in due course of time.

The male cockroach releases sperms into the female tract (fig. 2 6.7), the fertilized eggs are laid in cocoons (each containing 16 eggs). Different insects lay eggs differently.

In Higher animals like mammals. A special feature i.e. the ovarian cycle (menstrual cycle) occurs. In human female during the middle of each cycle an egg is released from one of the ovaries.

Ovaries alternate in releasing the egg. If sperms are present in the female tract, fertilization occurs (if not the egg perishes). Ovarian cycles vary in their duration and in the process a certain number of eggs are released, in different mammals (in humans normally only one)

Care of the young or of the egg is very well exhibited in many vertebrates specially in birds and in social insects (fig.26.8). Nest building, protection, feeding and even "instructing" the young ones to learn to face life is common in birds and in many other animals. In human beings the period of parental care is much prolonged.

Parthenogenesis is the process in which an individual develops from an unfetilised egg. In some insects like honey bees and wasps, the eggs fertilized by sperms develop into males which are haploid.

• Reproduction counter balances death and provides continuity of organisms in nature.
• Reproduction requires some sort of a reproductive unit. Such units could be large parts of the body (vegetative parts), specialised units (like spores and buds) or specialised cells (gametes).
• Regenerated reproduction often requires injury cut as a stimulus.
• Single-celled animals and lower forms like Hydra repoduce by fission and budding.
• Sexual reproduction is characteristic of all vertebrates and it usually involves sexual dimorphism.
• Fertilization and development may be internal or external.
• Number of eggs laid is related to the extent of survival chances of the young. More risks more eggs.
• Animals may be oviparous, viviparous or ovoviviparous.
• Major advantage of asexual reproduction is to produce the same variety in large numbers, while that of sexual reproduction is to produce variety in nature.
• Higher forms like humans, show reproductive cycles in females.
• Parthenogenesis, courtship and parental care are also some interesting aspects in animal reproduction.

1. (a) survival

(b) 16, 13-15

(c) Hot, Cold

2. (a) Because it needs to grow first and attain maturity.

1. Four cuts

2. Amoeba, Hydra, sponge, starfish.

(b) Zygote, fusion.

1. Earthworm/Hydra

2. Cocoon

3. One

2. 340
3. Large

2. No variety and no evolution.

3. Developement of an individual from an unfertilized egg. •