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| Summary |
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| 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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| The male reproductive system consists of Testis, epididymis, vas deferens, ejaculatory duct glands and penis. The female reproductive system consists of 2 ovaries, a duct system of 2 fallopian tubes, uterus and vagina. |
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| Spermatogenesis takes place in the seminiferous tubules of testes and oogenesis takes place in the graafian follicles of ovaries. Each primary spermatocyte gives rise to 4 sperms. A human spermatozoan is a long flagellated motile cell having a head, a middle piece and a tail. Each primary oocyte gives rise to a single ovum and 3 polar bodies. |
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| The menstrual cycle consists of cyclic changes in the reproductive tract of the primate female, culminating into a menstrual flow of blood from the vagina. The first half of the phase and the changes in these phases are controlled by estrogens and progesterone. The secretion of these hormones occur cyclically because of the cyclic secretions of pituitary gonadotropins in the female. |
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| The fertilised ovum in mammals and other higher animals passes through specific stages of development to attain the adult organisation comprising millions of cells. The totality of developmental events which transform a fertilised egg into an adult form is called embryonic development. |
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| The successive preparatory and actual stages of development include |
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 the formation of gamete |
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 the fusion of male and female gametes |
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 rapid division of the zygote to form a sphere or disc of cells |
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 reorientation of cells to form 3 germ layers |
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 differentiation of cells to form structures and organs. |
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| Fertilisation is the process of union of an ovum with the sperm. It is a physio - chemical event. Only one sperm gets entry into the ovum. After fertilisation, the fertilised ovum trickles down into the uterus for implantation. |
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| Cleavage is the process which transforms the fertilised ovum into a sphere of closely aggregated cells. The fertilised ovum divides repeatedly in close succession to form blastomeres. Cleavage divisions do not bring any appreciable increasse in the mass of protoplasm in the developing embryo, but there is a marked increase in the chromosomal DNA contents of its cells. |
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| Due to the rearrangement of blastomeres, a central cavity is formed inside the morula, called blastodermic vesicle. The outer layer of the blastodermic vesicle expands into the trophoblast which eventually develops into the foetal portion of the placenta. |
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| An internal cluster of cells called the inner cell mass, aggregates at one pole of the distended blastodermic vesicle. The vesicle gets implanted in the inner wall of the mother's uterus with the help of the placenta. |
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| Gastrulation is the visible dynamic event of cell movements. The cells of the blastula move in small masses or in sheets to reach their final locations. The cells of the blastula thus gets differentiated into 3 germ layers - ectoderm, endoderm and mesoderm. Each of the three germ layers is destined to develop specific organs and organ systems of the developing animal. |
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| The establishment of the germ layers starts the phase of differentiation and specialisation. The localised cell groups of the germ layers are sorted out in an orderly fashion with unique precision. Thus, the structures and organs of the animal body gradually take their characteristic shapes. |
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