In an experiment, a researcher colored a bit of tissue on the outside of a frog gastrula with a fluorescent dye. The embryo developed normally. When the tadpole was placed under an ultraviolet light, the glowed bright orange.

Brain

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.

Brain

Explanation:

Basically biofluorescence is one of the characteristic features of Amphibians. They have ability to emits light lays when shown on their tissues.Fluorescent dyes are used as tracers in living tissues as used in this experiment, so that when UV is shown on the dye-absorbed tissues, it glow lights.

Evidently B-vitamins are capable of absorbing UV-lights, and emits glow  of lights  on exposure.The brain-through its neurons is one of the  highest local metabolisms of B-vitamins  for neuronal functions.

Therefore when the UV-lights was shown on the amphibian, the B- vitamins in the brain tissues, with the absorbed the dyes absorbed   UV -lights to give orange glow.