Abstract
1. 1. When eggs of the sand-dollar Dendraster excentricus are cultured in concentrations of bromodeoxyuridine (BUDR) above 1 × 10 − M, cleavage produces a disordered mass of cells, but a blastula is not formed. The effect is completely antagonized by an adequate concentration of thymidine. 2. 2. The developmental abnormality is attributed to a disturbance of the separation of sister chromosomes at anaphase. Chromosome bridges are formed, which persist as the cleavage furrow divides the cytoplasm. As the interphase nucleus is organized from the two bridged groups of chromosomes, it takes the apparent form of a single nucleus extending through the furrow as a nuclear bridge. At the next prophase, the chromosomes in the bridges recondense in extended form in which they came out of the previous division. The original bridges are broken at this next division, as new bridges form through the new furrows. The developmental abnormalities are attributed to the abnormalities of the distribution of chromosomes. 3. 3. In eggs of several species of sea urchins, the nuclear bridges are not formed as a rule, and early development to the blastula stage is normal. 4. 4. Hypotheses relating the chromosome separation to the properties of DNA which has incorporated BUDR are considered.
Published Version
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