Histogenesis of lymphoid organs in larvae of the South African clawed toad, Xenopus laevis (Daudin).

Before embarking on a study of the lymphoid system of an amphibian larva, it is necessary to realize that lymphoid tissue may occur in many organs of the body and that in all immature vertebrates and in adult poikilotherms separation of lymphoid tissue from myeloid tissue is incomplete (Yoffey, 1960). Jordan (1938) reviewed the early work on the haemopoietic tissues of Amphibia. Cooper (1967tf, b) and Baculi & Cooper (1967) made a thorough investigation of the lymphomyeloid and lymphoepithelial organs of the bullfrog, Rana catesbeiana, in the branchial region of the larva and in the ventral neck region of the adult. In the larva they found three pairs of ventral cavity bodies and one pair of lymph glands; in the adult, propericardial bodies, procoracoid bodies, epithelial bodies and jugular bodies were present. Cooper (1967a) reviewed the literature on these organs and discussed their nomenclature. The ontogenetic development of the thymus of Rana catesbeiana was described by Cooper (19676), who also discussed its role in immunity. Recent work on larvae of the midwife toad, Alytes obstetricans, has shown that lymphoid tissue occurs in the thymus, spleen, dorsal coelomic lymphomyeloid organs, liver and kidney and is also associated with the mesentery, gut, gills and lungs (Du Pasquier, 1968). There is relatively little lymphoid tissue associated with the post-pharyngeal gut epithelium in most amphibians; there are no Peyer's patches and no bursa of Fabricius (Jolly, 1923; Good, Finstad, Pollara & Gabrielsen, 1966; Good, Finstad, Gewurz, Cooper & Pollara, 1967). However, Du Pasquier (1968) found many small lymphocytic infiltrations in the intestine of Alytes obstetricans larvae and also a large oesophageal lymphoid organ. The present paper describes the lymphoid organs of Xenopus laevis larvae and the sequence of their development. The work was undertaken as a preliminary investigation to our experimental studies on the ontogenetic development of certain immunological responses. The rationale is similar to that of Hildemann & Haas (1962) who described the order of appearance of different types of circulating blood cell in newly hatched and young Rana catesbeiana