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Reproduction and Development

Introduction

     An animal performs all its life functions with an aim to be alive till it matures and reproduces. Reproduction is the ability of living organisms to produce new individuals similar to themselves. Reproduction ensures perpetuation and survival of their species. It leads to an increase in the number of individuals of a species when conditions are favourable.

Types of asexual reproduction

     Production of offsprings by a single parent without the formation of gametes is called asexual reproduction. It is also called as somatogenic reproduction.

Different Types of Asexual Reproduction

     Binary fission involves mitosis only and hence the resultant individuals are genetically identical to each other and to the parent.

Advantages and Dis-Advantages of Asexual Reproduction

     There is no need for sexual partners. No chance for combinations, so no variation in the population.

Sexual Reproduction

     Sexual reproduction involves the formation and fusion of special sex cells called gametes. It is a complex process.

Types of Sexual Reproduction

     It is the complete and permanent fusion of 2 haploid gametes to form a diploid zygote.

Basic Processes involved in Sexual Reproduction

     Sexual reproduction basically involves 2 steps for maintaining a constant chromosome number in every generation.

Male Reproductive System in Human

     In human beings, sexual reproduction takes place. Sexes are separate and hence they are known to exhibit sexual dimporphism. Males have sex organs called the tetes while the females have sex organs called the ovaries. Even in the external appearance, males and females show differences.

Female Reproductive System in Human

     The female reproductive system is associated with the formation and maturation of the egg and the feeding and protection of the developing embryo inside the womb.

Gametogenesis

     Sexual reproduction involves the fusion of two haploid gametes to form a diploid individual.

Spermatogenesis

     The process of spermatogenesis occurs in the seminiferous tubules of the testes.

Structure of Gametes - Spermatozoan

     A sperm is a haploid male gamete which fuses with the ovum to form the diploid zygote.

Ovum

     The ovum is the female haploid gamete which fuses with the sperm and develop into an organism after fertilisation.

Menstrual Cycle in Women

     In women, reproductive phase begins with the onset of menses at about the age of 13 years. It ends with its cessation (menopause) at about the age of 45 - 49 years. During this reproductive period, the ovaries and the female reproductive tract undergo a series of cyclic changes which are primarily meant to prepare them for fertilisation and pregnancy and collectively form the menstrual cycle.

Hormonal Control of Cyclic Changes in Ovaries and Uterus During Menstrual Cycle

     The cyclic changes in the ovary and uterus are under the control of estrogen and progesterone hormones whose secretion is under the control of gonadotropic hormones secreted by anterior pituitary.

Menopause

     It is the period when ovulation and menstrual cycle stops in human female. It sets in about 45 - 55 years.

Embryonic Development

     All the events which change a zygote into a fully developed organism is called embryonic development. The entire process of development is called embryogenesis. Von Bear is called the "Father of Modern Embryology".

Steps of Fertilization

     As a result of copulation, semen containing sperms is ejaculated into the vagina. Once the semen (mixture of spermatozoa and accessory fluids) is deposited into the vaginal passage of the female, the spermatozoa start their journey through the uterus to reach the oviducts. On an average, the spermatozoa can swim several millimetres each second. Apart from this speed, they need to increase their mobility.

Cleavage (Segmentation)

     Cleavage is an unique embryological process which transforms the single fertilised egg cell into a sphere of closely aggregated multitude of cells. Immediately after fertilisation the fertilised ovum (egg cell) undergoes a series of cell divisions. These divisions are called cleavage divisions, which are all mitotic divisions.

Gastrulation

     Gastrulation is an important dynamic process in the development of the zygote which involves movement of cell masses from the surface of the blastula to their definitive positions in the embryo and their three primary germinal layers i.e., ectoderm, endoderm and mesoderm. These movements are called as morphogenetic movements.

Placenta

     Placenta is the mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.

Summary

     Animals may reproduce by asexual and sexual modes. The reproductive system of sexually reproducing animals consists of primary sex organs which produce the gametes, the secondary sex organs which participate in reproduction but do not form gametes and accessory sex characters which distinguish the two sexes in appearance.



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