Competence of Larval Tissue or Multiple Responses

The responsiveness of different larval tissues to thyroxine is markedly different. The tissues of the tail becomes necrotic and undergo degeneration due to the action of the thyroid hormone, while the tissues of the limb increase and undergo differentiation. Furthermore thyroxine causes rapid aging and destruction of R.B.C's of the larva undergoing metamorphosis. In addition it stimulates the development of cells that synthesize the hemoglobin of adult frog. The hormone T₃ reduces biosynthesis of nucleic acids in the tail but increases their synthesis in the liver. Furthermore, muscles of the tail undergo degeneration while those of the trunk remain unaffected. Such examples clearly indicate that the response and reactivity of larval tissue to the same hormone or agent differs greatly. In addition it can also be demonstrated experimentally that the reactivity and response of the target tissues are intrinsic, specific and independent. This can be demonstrated by transplanting tail tips of the tadpole to the trunk of another tadpole undergoing metamorphosis, or by placing the eye of the tadpole in the tail of a metamorphosing larva. The extra tail transplanted into the tadpole host trunk is not protected from degeneration and undergoes necrosis along with the host tail and gets absorbed. The eye retains its integrity despite the fact that it lies surrounded within the degenerating tail.