[Morphometric study of the thymus of the urodelan amphibian Pleurodeles waltlii Michah; Histological study and cell counting (author's transl)].

Thymus morphogenesis was studied in semi-thick sections, in order to examine the relationships between the size and cell counts of the thymus, and the weight and length of the animals. Histological structure was also investigated. The growing phase extends over the whole larval life up to stage 55b : thymus size increases progressively. A positive correlation exists between the size of the thymus, and the weight and length of the animals. Cell counts increase rapidly, particularly between stages 46 and 53; cell density increases from stage 40 to stage 55a. At the same time the thymus acquires its mature aspect : it is then composed of two parts; a dense background and medulla-like spots, these spots being randomly distributed among the background, and consisting of a small number of thymic cysts. The second phase is that of dynamic stability, covering metamorphosis : thymus size, cellular density, and cell counts are maximal and quite constant. The stability period is longer for cellular density than for thymus size and cell counts; during period the histological aspect of the organ shows no change. In the third phase, extending during adult life, there is a slight decrease in thymus size and in the thymus cell count; cellular density decreases. The rate of decrease thymus size, cellular density and cell count is less rapid than their previous rate of increase.