Jan 9, 2015

Sexual Reproduction in Animals

Syngamy: This type of sexual reproduction involves the fusion of two gametes to form the zygote. Depending upon the source of fusing gametes, syngamy is of two types endogamy and exogamy.

(a) Endogamy: It is also called self fertilization. It involves the fusion of two gametes of the same parent. It is uni-parental and is not of common occurrence e.g. Taenia (tapeworm)

(b) Exogamy: It is also called cross fertilization. It involves the fusion of two gametes formed by different parents. It is bi-parental and is very common e.g. frog, rabbit, monkey, man etc.

Conjugation: It involves temporary pairing of two parents called conjugates to exchange their male pronuclei e.g. Paramecium.

Steps of Sexual Reproduction

1. Gametogenesis

Gametes are the reproductive units in sexual reproduction. These are of two types: male gametes and female gametes. The male gametes are called microgametes or spermatozoa and female gametes are termed as macrogametes or egg or ova (sing. ovum). The male gametes are usually small and motile, so as to swim towards the female gamete for fertilization. The female gametes are large non-motile and laden with food to nourish the developing embryo. The formation of haploid gametes from diploid germ cells in the gonads for-sexual reproduction is called gametogenesis.
Spermatogenesis is the formation of sperms in the testes and oogenesis is the formation of ova in the ovary.
Mating: It is the sexual union to facilitate the reception of sperm by a partner female or copulation (Latin: copulatus means join).

2. Fertilization

The fusion or union of male and female gamete is called fertilization. It may be external or internal.

External fertilization: Male and female shed their gametes at the same time and at the same place in surrounding water. Fertilization of egg takes place in water e.g. Obelia, Nereis, Labeo, Frog.

Internal Fertilization: Sperms are passed from the male into the female body with an intromittent organ, the penis. The development may take place outside the body e.g. shark, lizard, cuttlefish or inside the body and the mother's body provides the warmth and nourishment to the growing embryo.

Comparison between External and Internal Fertilization

External Fertilization

1- Fertilization takes place outside the body.
2- It takes place in animals mostly living in water
3- The sperms, eggs are shed into the water at the same time and same
4- As most of the gametes are lost in water, animals shed large number of gametes.
5- Zygote formed, remains in water
6- Male and female may or may not come close together for external fertilization

Internal Fertilization

1- Fertilization takes place inside the body.
2- It takes place in animals living in land and in water.
3- The sperms are shed inside the body of the female.
4- Chances of fertilization is more so comparatively less number of gametes are shed.
5- Zygote formed may or may not be retained in the body.
6- Male and female must come close together for internal fertilization.

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