The development of the thymus bodies in necturus maculosus

The primordia of the thymus bodies in Necturus begin to form in 7‐mm. embryos. These structures arise as clumps of entodermal cells on the posterior dorsal portion of the first four visceral pouches; the first or last pouch, being rudimentary, fails to form such a primordium. All except the first of these primordia lose their connection with their respective pouches in the 16‐mm. stage and lie free in the surrounding connective tissue. The primordium on the first pouch begins to show degenerative changes during that stage, losing connection with its pouch in the 23‐mm. stage, finally to disappear altogether in 30‐mm. specimens. The three remaining bodies continue to grow, but the second and third outstrip the fourth in development and begin to form Hassall's corpuscles in 32‐ to 34‐mm. stages. Sections of 30‐mm. Necturi show a few of these corpuscles fairly well developed and containing a faintly staining secretion. The fourth body remains relatively small a long time, not showing the Hassallian cysts until the animal is 60 mm. long. By shifting of positions and increasing in size, the three bodies come to lie in proximity, one behind the other, to form the definitive thymus gland of the adult. The gland lies embedded in the heavy musculature on top of the head in front of the gills.