Publications from the Akkeshi Marine Biological Station;No.15


Studies on the Development of Tubularia radiata and Tubularia venusta (Hydrozoa)


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The development from the egg to the young polyp of Tubularia radiata and T. venusta is described. The developmental processes of the two species coincide in principle with each other. The cleavage is total and vertical until the fourth division , but some eggs of T. venusta cleave very irregularly and a syncytium is f ormed . The blastula is a modified coeloblastula. The gastrulation takes place, at first, by multipolar proliferation or by cell divisions like an inward proliferation and a solid mass of cells is formed, then the primary germ layers are differentiated by further cell divisions like a delamination. Finally the separation of the germ layers is completed by migration of the interstitial cells toward periphery, which have been produced by unequal divisions of the primary endoderm cells. The embryo becomes flattened along the future body axis and conical rudiments of aboral tentacles protrude. Then the growth of body along the oralaboral axis begins and the coelenteron is formed. Later , the embryo develops gradually into actinula by growth along the body axis and by elongation of the aboral tentacles. The internal changes such as simplify of future mouth area , development of fixing cells at aboral end, formation of endoderm cushion and others are also observed. Each characteristics of the actinulae of the two species seem to be adapted to their habitats. The comparison with other species and the systematic discussion from the developmental view are made.