Question

Tadpoles of frog can be made to grow as giant size tadpoles, if they are
A. Administered large amount of thyroxine
B. Administered antithyroid substance like thiourea
C. Reared on a diet rich in egg yolk
D. Reared on a diet rich in both egg yolk and glucose

Answer 1

Hint: The larval stages of development in the life cycle of an amphibian is the tadpole stage. It is an aquatic organism with various features that is absent in the adult amphibians. As the tadpoles grow into frogs, these features are either lost or undergo several types of changes.

Complete answer:
The tadpoles represent the larval stages of the amphibian species of frog. They are known to bear several types of specialized features that help them to grow in aquatic conditions. The tadpoles consist of a lateral line, gills for breathing in water and tails.
The tadpoles grow into adult frogs through a process of development which is known as metamorphosis. In this process, the tadpoles undergo several abrupt changes to convert into an adult frog. In amphibians, metamorphosis is regulated by the action of hormone thyroxine. Thyroxine produced by the thyroid glands is responsible for the changes related to metamorphosis.
Thyroxine and iodine stimulate the process of metamorphosis and leads to apoptosis of the cells of the larval characteristics. Iodine acts as a limiting factor in the process of metamorphosis. It also activates the hormone thyroxine which eventually brings about metamorphosis. The tissue differentiation in the gills, tails and fins that are present in the tadpoles as well as the size of the tadpoles are affected by the secretion of thyroxine. It also stimulates the changes of the nervous system transforming the aquatic tadpole into terrestrial frogs.
Thiourea is an organic compound containing Sulphur, nitrogen, hydrogen and carbon. It is similar in structure to urea except the oxygen atom is replaced by a sulfur atom. Thiourea acts as an inhibitor agent in the process of metamorphosis of tadpoles. In other words, the thiourea acts as an antithyroid substance which interferes in the thyroxine action of metamorphosis. It results to cause giant size development in the tadpoles and they may remain indefinitely in this particular stage only.

Hence, the correct answer is option (B).

Note: In frogs, the tadpole stage is a larval stage which undergoes metamorphosis. Through the process of metamorphosis, the tadpoles undergo several changes and these changes are stimulated by the cation of thyroxine hormone. In this case, if some antithyroid substances such as thiourea are administered, it causes the tadpoles to grow into giant sized tadpoles as it interferes with the normal process of metamorphosis.