Sexual Reproduction in Animals

  Animals reproduce to continue their race. Animals reproduce both by asexual and sexual methods. Asexual reproduction occurs in the organisms of lower phyla like protozoa, coelenterate, porifera etc., Majority of the animals adopt sexual reproduction. Male sex organs are called the 'Testes' while the female sex organs are called the 'Ovaries' and they produce cells called 'Gametes' like spermatozoa (or) sperms and ova respectively.
          Fusion of gametes is called fertilization. Fertilization is of two kinds. When fertilization occurs inside the body of the female animal is called internal fertilization and when fertilization occurs outside the body of the animal, it is called external fertilization. In majority of the animals, sexes are separate and male and female animals are separate. This is called sexual Dimorphism and animals are said to be unisexual.
Conjugation in Paramoecium:
            Paramoecia, after repeated binary fissions, become weak and switch over to sexual reproduction known as conjugation.
The Paramoecia that undergo conjugation are called conjugants. The two conjugants come close and unite. The pellicle that separates them dissolves. The cytoplasm of the two paramecia becomes continuous through this part, where the pellicle is dissolved. This is called as cytoplasmic bridge. At this stage the macronucleus slowly disappears and the micronucleus undergoes meiotic division resulting in four haploid nuclei. Of these four nuclei, three disintegrate and the rest single micronucleus divides into small migratory (male) pronucleus and large stationary (female) pronucleus.
             The male pronucleus of one conjugant fuses with the female pronucleus of another conjugant through cytoplasmic bridge resulting in synkaryon. Then after the conjugants get separated and are called as exconjugants.
In each exconjugant, the synkaryon undergoes 3 mitotic divisions and form 8 nuclei. Of these eight, three disappears. Of the remaining five nuclei, four develop into macro nuclei and one into micronucleus.
              Each exconjugant, with a single micronucleus and four micronuclei divides twice and forms four daughter paramoecium. Each daughter paramoecium receives one macronucleus from the exconjugant.
              The single micronucleus of the exconjugant divides twice along with the exconjugant and each daughter paramoecium receives one micronucleus. Thus each of the daughter paramoecium receive one macro and one micronucleus. Thus, by conjugation a single paramoecium produces four daughter paramoecium
Different stages in the conjugation of Paramoecium

Reproductive System in Earth Worm
Earthworm is hermaphrodite - both male and female sex organs are present in the same individual. But, earth worm shows protandry condition that is the testes matures much earlier than ovaries and this prevents self fertilization. The testes are present one on each side of the body in 10th and 11th segments. They produce sperm mother cells which inturn matures into sperms in seminal vesicles. The sperms passes out through vas deferens and into male genital aperture.
          A pair of ovaries are present, one on each side of the ventral nerve cord. They arepresent in the 13th segment. Each ovary has one oviduct, which opens out through female genital aperture. Ovaries produce ova and the ova are sent out through an aperture.
A thick girdle like clitellum is formed in between 14-17 segments. When the ovaries mature, the clitellum secretes a viscous substance becomes sticky and attaches with the substratum.
This is called cocoon. Earth worm wriggles and retreats back leaving the clitellum. Eggs and sperm cells are released into this cocoon. Both the ends are closed and it gets hardened to form a cocoon. Fertilization occurs inside the cocoon.
Since fertilization is taking place outside the body, it is external fertilization. After about 8 to 10 weeks, young earthworms come out of the cocoon.

Reproductive System in Earth Worm

Cocoon Formation in Earth Worm

Reproduction in House Fly
In male fly, a pair of testes are present one on either side of the ventral cord. Testes produce sperms. Each testes is connected to a tube called vas deferens. The two ducts unite to form a single ejaculatory duct. It opens out through male genital opening. In female fly, a pair of ovaries are present in the abdominal segment. Each ovary is connected to a oviduct. The two ducts unite posteriorly to form uterus.
It opens out through female genital opening, and is located in a special structure called ovi positor which helps in releasing fertilized eggs. A pair of sperm theca are attached to the ovi positor. They store sperms received during copulation.
Fertilization is internal. The eggs are laid by the female from 6-8 weeks after copulation. The eggs are laid in a clustor in decaying organic matter like manure, cow dung, decomposing fruits and grass, human faeces etc.
Reproduction in House Fly
Reproduction in Frog:
Frog is unisexual and shows sexual dimorphism.
Male Reproductive system:
            The male reproductive system consists of a pair of testes, vas efferentia and a pair of urinogenital ducts. Testes is an oval and yellow body which is attached to a kidney. Inside the testes there are a number of coiled tubules called seminiferous tubules. Spermatozoa are produced in the tubules. Each testes opens into a kidney through 10- 12 tubes called vas efferentia. Spermatozoa enter the kidney through these tubes.
From kidney two tube-like structures called ureters enterinto a chamber called cloaca. The ureters in male frog are called urinogenital ducts.
Spermatozoa produced in seminiferous tubules of testis enter into kidney through vas efferentia and from kidney into cloaca through urinogenital ducts. Cloaca opens outside through cloacal aperture. The discharged spermatozoa is called milt.

Male Reproductive System in Frog
Female reproductive system of Frog:
           It consists of pair of ovaries and a pair of oviducts. A pair of lobed ovaries are attached to the kidneys. Ovary is a sac like structure which is greyish or blackish in colour. Inside the ovary, numerous chambers called "Follicles" are present. Each folliclegives a single ovum.
From each ovary long folded, tube like structure called 'oviduct' arises. This opens anteriorly into the body cavity through a ciliated funnel called Ostium.
Female Reproductive System in Frog

The funnel collects ova from the body cavity posteriorly. The oviduct is enlarged into a sac called ovisac or uterus. which stores ova. The oviducts open into cloaca and are discharged out through the cloacal aperture. Frog release a large number of eggs as a mass called Spawn.
The ovum of frog is spherical. In the upper part is the nucleus and this part is black due to pigment present in the cytoplasm. This part is called animal hemisphere. Yolk is concentrated at the other pole, which is whitish. This pole is called vegetal hemisphere.
Frog Ovum


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