Thymus gland develops from embryonic endoderm.

Explanation:

- The thymus gland is a specialized organ of the immune system that is responsible for the maturation and differentiation of T cells, a type of white blood cell that plays a crucial role in cellular immunity.
- During embryonic development, the thymus gland originates from the third and fourth pharyngeal pouches, which are outpocketings of the embryonic endoderm.
- The endoderm is one of the three germ layers that form during early embryonic development. It gives rise to the innermost layer of cells in the developing embryo, which will eventually differentiate into the lining of the digestive and respiratory tracts, as well as some other internal organs, including the thymus gland.
- The development of the thymus gland involves a complex series of interactions between the endoderm and other nearby tissues, including the neural crest cells and the mesoderm, which give rise to the surrounding connective tissue and blood vessels.
- As the thymus gland develops, it acquires a distinctive structure consisting of two main lobes connected by a central isthmus. Within each lobe, there are numerous lobules or follicles, each containing a network of specialized cells known as thymic epithelial cells, which play a critical role in the development and selection of T cells.
- Overall, the development of the thymus gland from the embryonic endoderm is a highly coordinated and tightly regulated process that is essential for the normal function of the immune system.