Studies of cleavage and development of isolated sea urchin blastomeres

Abstract

Isolated blastomeres from young larvae of Echinocyamus pusillus and Psammechinus miliaris were studied by means of time-lapse cinematography. The isolated mesomeres and macromeres cleave at about the same rate as in situ in intact larvae. It was found that single meso- and macromeres were able to develop into blastulae. Isolated micromeres cleave somewhat more slowly than the micromeres in situ and as isolated cells they were not able to pass through more than three cleavages. They are characterized by the sending out of cytoplasmic protrusions. The rate of cleavage is different for meso-, macro- and micromeres and the meso- and macromeres cleave, both in situ and as isolated cells, considerably faster than the micromeres. Owing to the different rates of cleavage during the blastula stage the cells of the prospective ectoderm multiply more rapidly than the cells, which later form the endoderm and mesoderm and therefore the early development is characterized by an “animal” trend. Even minor changes in the rate of cleavage, for instance a retardation of the mesomeres, may therefore later on in the course of differentiation result in so-called vegetalization.