REPRODUCTION IN TOAD
In sexual reproduction, the male animal produces the sperm while the female produces the egg or ovum. The fusion of the sperm and the egg in the fallopian tube of the female during mating is called fertilization. The fusion of these sex cells, i.e sperm and egg results in the formation of what is called zygote. The zygote undergoes cell division to form a developing organism called embryo.
STAGES IN THE DEVELOPMENT OF TOAD
a) Courtship stage: Reproduction in toad usually takes place during the rainy season. The males make loud croaking noises to attract the females which are usually swollen with eggs. The male toad later climbs on the female’s back and holds her firmly with the thick pads on its thumbs. As the female lays her eggs, the male releases its sperms on them resulting in external fertilization.
b) The egg stage: The eggs laid are surrounded in strings of jelly. The jelly performs the following functions:
- It protects the eggs from mechanical injury when water is disturbed.
- It separates the eggs to enable them receive adequate oxygen.
- It prevents bacterial and fungal attack on the eggs.
- It prevents the eggs from drying up. The fertilised eggs (zygote) undergo cell division by mitosis process to form the embryo. The embryo feeds and grow around the egg yolk. Dissolved oxygen diffuses through the jelly into the embryo.
c) The young tadpole stage: The series of changes which take place from the development of tadpole inside the egg to adult stage is called metamorphosis. After 1 to 2 days, the young tadpole emerges from the jelly and grow. At this time, it attaches itself to a water weed by a sticky substance secreted by a V-shaped cement gland on the underside of the head. At this stage, the tadpole has no mouth and still feeds on the egg yolk. It obtains dissolved oxygen through the skin for respiration. Developing eyes, ears and nostril are seen and a portion marked for the appearance of the mouth is equally noticed.
d) The external gill stage: the young tadpole now has a clear head, body and tail. It develops three external gills on either side of the head for respiration. It also develops horny jaws which are used to feed on water weeds. The intestine is long coiled. The V-shaped cement gland is still present in the tadpole.
e) Internal stage: About 6 – 10 days after hatching, the external gills disintegrate while the internal gills develop. A gill cover (operculum) grows over the gills leaving an opening called spout on the left side only through which water flows out of the gill chamber. The tadpole now breathes like a fish. during this stage, the tail elongates and muscles then develop.
f) The limb stage: The hind limbs grow first and develop. The fore limbs then start to grow out. The left fore limbs appears first through the opercular opening, followed by the right which disintegrate while the lungs start developing. The mouth develops and replaces the horny jaws. The intestine shortens, the tadpole starts eating small small animals in the water. The eyes become big and prominent.
g) Young toad stage: The tadpole changes into a small toad by reabsorbing the tail. It comes out of the water to the land where it grows into a full adult toad. It usually takes about 40 – 45 days for metamorphosis in toad to take place, i.e. from egg to adult toad.
Effects or Role of Hormone in the Development of Toad
In toad and other amphibians metamorphosis is controlled by the hormone called thyroxine. It is produced by the thyroid gland in the region at the junction of the head and the trunk. For thyroxine to be made, there must be iodine in the water. The tadpole cannot change into adult form in the absence of this hormone. However, an increased level of thyroxine hastens metamorphosis.