Determination of early cleavage blastomeres in the sea star, ASTERIAS FORBESI
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Abstract
Determinative events of early echinoderm blastomeres has been mostly studied in sea urchins (class Echinoidea) because they are excellent subjects for experimental embryological investigations. Comparatively, little is known in sea stars (class Asteroidea) though they are also good subjects for these investigations. We have isolated two-, four-, and eight-cell stage blastomeres and fused early cleavage stage embryos of the sea star Asterias forbesi Desor. Generally, isolated blastomeres took longer to develop than the control embryos. Fully differentiated blastomeres were evaluated as: "normal " larvae, "abnormal" larvae, animalized, and vegetalized. "Normal" larvae derived from the same embryo varied in morphology. A low percentage of all blastomeres isolated from a single embryo of the two-cell stage (22%) or the four-cell stage (22%), differentiated into "normal" dwarf larvae. Overall, a low percentage of any blastomeres isolated from the two- or four-cell stages formed "normal" dwarf larvae. There were no examples of blastomeres isolated from a single eight-cell stage embryo which formed an entire set of "normal" larvae. Five sets (8%) formed four "normal" larvae and four "abnormal" larvae. These results are explained in terms of their relationship to the presumed axial determination of the embryo, and compared to results of other similiar experiments performed by Schaxel (1914) and Dan-Sahkowa and Satoh (1978).
Fused embryos of either two- or four-cell stage have the capacity to form single larvae, including harmonious larvae. No conclusions could be drawn on orientation and cell stage of the fused embryos and how they may relate to the results without further investigation.
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Thesis (M.A.)--Boston University
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