Abstract:betweenlateembryoandlarvalstageswasstudiedbymicroscopicobservationofserialsections.Atawatertemperatureof10.0–16.2°C,theorderofappearanceoflymphoidorganprimordiawasthymus,headkidneyandspleen.Theprimordiumofthethymusformed5dbeforehatchingandsmalllymphocytesappeared1dbeforehatching.Nineteendaysafterhatching,connectivetissueandbloodcapillariespenetratedintotheparenchymaofthethymusandthetrabeculaeappeared.Differentiationintoindistinctinnerandouterzonesoccurredafter35d.Theheadkidneyprimordium,whichconsistedofundifferentiatedhemopoieticstemcells,appeared2dbeforehatching.Kidneytubuleswereformed3dafterhatchingandlymphoidorgansafter10d.Between35dand40dafterhatching,theepitheliumoftherenaltubulesbegantodegradeandbloodsinusesappearedintheheadkidney.Whentherenaltubuleshaddegeneratedcompletely50dafterhatching,theheadkidneywascomposedprincipallyoflymphoidtissueandalargenumberofbloodsinuses.Thespleenprimordiumappeared5dafterhatchingandtheorganbecamelymphoidonthe16thday.Capillariesandbloodsinuseswereformedinthespleenbetween35dand50d.Weconcludethattheimmuneorgansaredifferentiatedearlierinthaninotherfishes.Althoughtheoveralldevelopmentratewasrelativelyslowin
