Answer:
Brain
Step-by-step explanation:
At the initiation of the gastrulation in the tadpole the tissue layer of the embryo break into three layers namely: the endoderm, the mesoderm, and the ectoderm.
The endoderm is the inner layer of the embryo tissue, and it is responsible for the formation of the digestive and the respiratory system of the tadpole. The mesoderm is the next layer after the endoderm, it forms the reproductive, circulatory, muscular and skeletal system of the frog. The ectoderm is the outermost layer of the embryo tissue which helps in the formation of the nervous system and the brain of the tadpole.
In the experiment, the tissue colored was the ectoderm that forms the brain. Therefore the brain glowed bright orange when the tadpole was placed under ultraviolet light.