EMBRYO DEVOLOPMENT OF RABBIT

After ovulation, secondary oocyte is taken into the fallopian tube (oviduct) where it fuses with a sperm received during copulation.
Thus fertilization is internal.
An unusual feature is that an entire sperm enters the cytoplasm of oocyte.
Tail of sperms however, soon disintegrates.
The haploid male and female pronuclei then become fused to form a diploid zygote nucleus.

The development of rabbit starts with diploid zygote or egg.
The embryo develops within a specialized part of the oviduct called uterus.
The embryo establishes an intimate contact with it, and the young are born alive so that the eutherian mammals (rabbit) are viviparous.

The egg of rabbit is 0.1 mm in diameter and is a secondary ooctye.
When shed from the Graafian follicle of the ovary, it is surrounded by a striated secondary membrane, the zona pellucida secreted by the follicle cells.
Around the zona pellucida are some follicle cells forming a corona radiata which is soon dissolved away.
The egg is alecithal with almost no yolk, the nucleus lies in the upper half or animal hemisphere, but the lower half or vegetal hemisphere has some particles of yolk distributed evenly in the cytoplasm.
The egg is fertilized in the upper part of the oviduct and it immediately undergoes a second maturation division to become a mature ovum.

Two polar bodies lie within the zona pellucida near the animal pole.
In mammals the entire spermatozoan penetrates the egg but the tail soon degenerates, penetration of sperm into an egg is brought by an enzyme hyaluronidase present in the semen.
The zygote passes down the oviduct and is covered by a coat of dense albumen.

Clavage is holoblastic but unequal.
The first cleavage furrow is vertical dividing the zygote completely into two unequal blastomeres, one being smaller and more opaque than the other.
The second cleavage is also vertical and at right angle to the first, so that zygote is divided into four unequal cells called blastomeres.
The third cleavage is horizontal and slightly above the equator. After this, cleavages are irregular.

Cleavage results in the formation of a more or less spherical solid mass or ball of cells called morula.
A fully formed morula shows an outer or superficial layer of cells, the trophoderm or trophoblast.
The morula stage passes down the oviduct to enter the uterus.
It makes contact with uterus of mother and absorbs liquid food.

The embryo swells rapidly and the liquid food collects in a cavity which separates an outer layer of smaller trophoblast cells from a solid inner cell mass.
The embryo is now called a blastocyst which appears like a blastula but is not comparable to it.
Those trophoblast cells which lie above the innercell mass are called cells of Rauber.

As the blastocyst grows the inner cell mass becomes a knob-like thickening at one pole and it is called an embryonal knob because all parts of the embryo will be derived from it.
The trophoblast cells opposite the embryonal knob form small papillae of trophoblastic villi which penetrate into depressions or crypts in the uterine wall.
The blastocyst becomes attached or implanted in the uterus of the mother.
The villi absorb food from the uterus.

Some cells separate from the lower surface of the embryonal knob.
These cells migrate and form the new layer of cells, the endoderm.
The blastocyst is now two cell layers thick like a gastrula. It encloses a fluid - filled yolk sac which contains no yolk.
The embryonal knob enlarges and becomes flat to form an embryonal disc.

It soon becomes exposed by disintegration of the cells of Rauber and by the zona pellucida and albumen being dissolved away.
This gastrulation is not comparable to that of Amphioxus or frog.
In the centre of the embryonal disc, cells collect to form a primititve streak.
The movements of the cells of the primitive
streak on both sides result in the separation of mesoderm cells.
Cells move forward and the primitive knob to form a notochord.
Formation of mesoderm and notochord is the same as in chick.
The primitive streak retreats towards the hind end.
After giving up the chordamesoderm cells of the notochord and mesoderm, the remaining cells of the embryonic disc form ectoderm.
The ectoderm cells form a neural plate and neural folds which becomes a neural tube.

The period of intrauterine development of foetus in mother is called gestation period.
The uterine-endometrium (vascular cushion for the implantation of the embryo) is maintained by a hormone called progesterone.
The gestation period in mammals varies from species to species.
The gestation period of different mammals are :

1. opposum - 13 days,

2. rat - 21 days,

3. rabbit - 30 days,

4. cat and dog - 60 days,

5. guinea pig - 68 days,

6. goats and sheep - 149 days,

7. cow - 280 days,

8. horse - 336 days,

9. whale - 360 days,

10. Rhinoceros - 540 days,

11. elephant - 600 days

12. man - 270 days.

Animals which give birth to theiryoung ones are called viviparous animals.
In these animals the development of the fertilized egg takes place in the uterus of the mother.
The development of the embryo depends on the mother for both nourishment and shelter.
This phenomenon of giving birth to young ones is called viviparity.
Fertilization is internal in these group of animals.

Ex: Snakes like vipers and all mammals except the egg laying ones.

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