The development of isolated blastomeres in Bithynia tentaculata (prosobranchia, gastropoda)

Abstract

The development of isolated blastomeres, separated at the first and second cleavage has been studied. After separation at the first cleavage, one half-embryo (CD) becomes swollen and balloon-like and develops adult organs such as eyes, tentacles, shell, operculum, etc. The other half (AB) is much more compact and never develops these adult structures. After separation on the alternate axis at the second cleavage to produce AD and BC halves, both halves could form all adult structures. These results indicate an equivalent organizing capacity of the C and D quadrants contrary to most spiralians, such as Ilyanassa , where the D quadrant alone exerts the major organizational influence. The experimental evidence is discussed in relation to the distribution of the cytoplasmic vegetal body, rich in RNA, which is passed to the CD blastomere in the polar lobe and then evidently equally distributed to C and D.