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In this article we will discuss about:- 1. Breeding of Toad 2. Fertilization of Egg in Toad 3. Embryonic Development 4. Larval Stage 5. Metamorphosis.
Breeding of Toad:
Toads breed during the rainy season. Males come by the side of a suitable pond or ditch and begin to croak loudly by inflating their vocal sac. Invited by the song, the females come to the water in a large number. This is followed by mating or sexual reproduction.
A male clasps a female, his grip on her being tightened by the thumb pads which are particularly well-formed during the breeding season. This state may continue for two or three days. The female how lays her eggs or ova in shallow water and the male pours his milt or spermatozoa over the eggs as they are extruded.
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The eggs look like small black spots placed upon white strings of jelly, thus forming a convoluted ribbon which floats freely on the surface of the water. This is the toad spawn. The spermatozoa swim actively by lashing their tail and a swarm of them encircle every egg on the spawn.
Fertilization of Egg in Toad:
Each egg is fertilized by a sperm, the process taking place externally, that is outside the body, in water. After the entry of one sperm, the outer membrane of the egg becomes impervious to other sperms. The sperm which has succeeded to enter the egg loses its tail; its head is now transformed into the male pro-nucleus.
In the meantime, the nucleus of the egg changes into the female pro-nucleus. In fertilization the male and female pro-nuclei fuse to form a single nucleus. The fertilized egg is known as the oosperm or zygote. It is a single cell which is destined to develop into a toad. Under abnormal circumstances, unfertilized eggs may develop by parthenogenesis and behave like fertilized eggs.
Embryonic Development in Toad:
Zygotes are left in water without and parental care. Each zygote undergoes a period of rest and then begins to develop by repeated cell division. The first cleavage results in two cells or blastomeres. A large number of blastomeres are rapidly formed, all enclosed within the outer membrane of the egg.
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The blastomeres arrange themselves into groups in a complicated way and eventually produce a young embryo with three germinal layers:
(1) An outer ectoderm,
(2) An inner endoderm, and
(3) A mesoderm in between the two.
The embryonic development goes on for about two weeks. At the end of this period a small embryo is seen to wriggle within the egg. An embryo may be defined as the developing young of an animal inside the egg. During the course of its development the embryo draws its nourishment from the yolk, which is stored in the egg for this purpose. Eventually the embryo hatches out by rupturing the egg membrane.
Larval Stage of Toad:
The freshly hatched young tadpole bears no resemblance to its parent. It has an ovoid head, a short trunk, and a slender compressed tail with a small vent near its root. There is no limb, but there is an adhesive sucker on the ventral side, by which it attached to some submerged object.
As there is no mouth it cannot ingest any food and depends on the residue of yolk. Three pairs of branched processes, called external gills, develop on the sides of the head. These are highly vascular folds of skin which serve as the first set of respiratory organs.
After resting for a few days a mouth is formed near the sucker and a pair of horny jaws encircle the mouth. The tail grows longer and develops a dorsal and a ventral fold. It is thus converted into a fin. Moreover, V-shaped bands of muscles appear on both sides of the tail which are now used for active swimming.
The free-swimming young scrapes algae with its jaws and ingests small bits of water-weeds through its mouth. Its alimentary canal grows enormously long, and to accommodate the same within the short trunk, the intestine is coiled spirally like the spring of a watch.
Evidently the newly-hatched young, though self-supporting does not resemble the toad either in form or in habits. It is known as the tadpole larva. A larva is defined as the self-supporting immature young of an animal which bears no resemblance to the adult.
At a later stage, the pharynx becomes perforated at the sides by gill-slits. Internal gills are now formed in between the gill-slits. The internal gills are vascular outgrowths from the pharyngeal wall. A fold of skin called operculum appears and covers up the gills and gill-slits. The operculum or gill-cover fuses with the trunk ventrally and on the right side, leaving a small opening, called spiracle, on the left side.
Water enters through the mouth and while passing out through the pharyngeal gill-slits washes the internal gills and finally escapes through the spiracle. The internal gills are the second set of respiratory organs of the tadpole and with their formation the external gills wither away. At this stage, the tadpole is not only fish-like in appearance but it also resembles a fish in the manner of locomotion and respiration.
When the internal gills have been functioning for a time, a pair of lungs appear as outgrowths from the ventral surface of the pharynx. The lungs are the third set of respiratory organs of the tadpole. Meanwhile, the limbs have been developing as buds. The hind limbs appear, one on either side of the root of the tail. The forelimbs remain under cover of the operculum for a time and then emerge by bursting through it.
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The tadpole now comes to live in shallow water. It can still breathe through the internal gills and comes to the surface occasionally for taking in a gulp of air into its newly-formed lungs. When the lungs are fully formed the internal gills rapidly disappear and the tadpole now looks like a miniature toad with limbs and everything but with a tail.
As the limbs grow in size the animal enters into a period of fasting and the tail is gradually absorbed into its body, the material of the tail being used as the source of nutrition. The young toad now leaves its watery home and hops on the land.
Finally the mouth widens and the horny jaws are replaced by true bony jaws. The animal changes its food and becomes a carnivore; consequently its gut becomes short and more or less straight. The development is completed in about twelve weeks.
Metamorphosis of Toad:
The young tadpole larva which hatches out from the toad’s egg hardly resembles its parents. It is more like a fish than like a toad and lives a free-swimming independent life. Though immature, it can take care of itself and procure its own food. The term metamorphosis is applied to the series of changes which a larva has to undergo for altering its structure and mode of life before it comes to resemble its parents.
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During metamorphosis of the tadpole, fish-like characters are given up and toad-like characters are taken on. The larva is thoroughly changed in its mode of respiration, locomotion, and nutrition. The metamorphosis occurs rapidly towards the end of the tadpole’s aquatic life. There is, however no abrupt transition from one stage to another.
Metamorphosis is hastened if tadpoles are fed artificially with the substance of the thyroid gland. In the American bull-frog the normal larval period is about two years; experimental feeding with thyroid reduces this long larval stage to a very short period.
On the other hand, removal of thyroid from a tadpole completely stops metamorphosis although the larva may grow and attain a huge size. Anterior lobe of the pituitary body plays an important part in the metamorphosis by stimulating the thyroid gland.
