This being the case, it is much more reasonable to interpret the thymus as an ontogenetic (embryonic), rather than a phylogenetic (racial) rudiment. It has been observed that, in the case of reptiles which lack definite lymphatic glands (which function in man as formative centers of lymphocytes or white blood corpuscles), the thymus is extraordinarily developed and abounds in lymphoid cells. It has also been observed that the formation of lymphocytes in the lymphatic glands is regulated by the digestive process; for, after digestion, the activity of these glands increases and the formation of leucocytes is accelerated. Since, then, the lymphatic glands appear to require the stimulus of the digestive process to incite them to action, it is clear that in the fœtus, which lacks the digestive process, the lymphatic glands will not be stimulated to action, and that the task of furnishing lymphocytes will devolve upon the thymus. After birth, the digestive process commences and the lymphatic glands become active in response to this stimulus. As the function of forming lymphocytes is transferred from the thymus to the lymphatic glands, the former is gradually deprived of its importance, and, in the interest of organic economy, it begins to atrophy, until, at the end of the child’s second year, or, at latest, when the child has reached sexual maturity, nothing but a reduced vestige remains of this once functional organ. “The thymus,” says Starling, “forms two large masses in the anterior mediastinum which in man grow up to the second year of life and then rapidly diminish, so that only traces are to be found at puberty. It contains a large amount of lymphatic tissue and is therefore often associated with the lymphatic glands as the seat of the formation of lymph corpuscles.... In certain cases of arrested development or of general weakness in young people, the thymus has been found to be persistent.” (“Physiology,” 3rd ed., 1920, p. 1245.)