EMBRYONIC-DEVELOPMENT OF POLYCLAD NOTOPLA NA-HUMILIS (TURBELLARIA, POLYCLADIDA)

Embryonic development of Notoplana humilis from the Japan Sea is descr ibed. Cytoplasmic blebs are formed at the animal pole during extrusion of the 1st and 2nd polar bodies. The cleavage is spiral and homoquadr antic. Cell-lineage is followed up to the 6th division, then the regul ar arrangement of cells at the animal pole disappears. At the animal p ole a transitory opening is formed. Four groups of trochoblasts have b een revealed at the early stages, though the larva has no ciliary lobe s. Gastrulation proceeds by epiboly. The blastopore is closed after fi nishing the gastrulation. Formation of brain and mesoderm was studied using histological methods, The polyclad pastille-like larvae hatched are supposed to be the secondary ones due to the presence of rudimenta ry trochoblasts at the early stages of embryogenesis.