Little is known about the biology of the Order Gymnophiona of the Class Amphibia. The organs of their immune system have been described only in adult specimens. We describe the development of the immune system in Typhlonectes compressicauda embryos from stages 24–33. At stage 24, just prior to hatching, the thymus and spleen develop from the fourth, fifth, and sixth visceral pouches. The spleen develops from a bud of cells in the mesentery, situated between the stomach and the mesonephros, and the thymus comprises a medulla and cortex. By stage 28, between hatching and metamorphosis, organelles resembling Hassal's corpuscles develop in the thymus, and are retained to the end of development and throughout adult life. At stage 30, just before metamorphosis, the spleen becomes organized into the red and white pulp. Upon disappearance of blood islets, lymphocytes appear to circulate between the spleen and thymus through the blood vessels. The kidney is never haematopoietic in either embryos or adults. The liver, which has been described as equivalent to the bone marrow, develops earlier than the thymus or the spleen, and becomes granulocytopoietic and monocytopoietic at stage 30. The development of the immune organs of Typhlonectes compressicauda is distinct from that of Anura and Urodela, although apparently closer to the Anura, because the spleen develops distinctly later than the thymus. The presence of a white and a red pulp in the spleen and of a medulla and a cortex in the thymus in early stages of differentiation indicate a highly evolved immune system in this species.
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