No abstract

Chronological changes of the chromosomes upon feritilization of the giant freshwater prawn, Macrobrachium rosenbergii (de Man), were followed. Egg chromosomes, within 2 min after spawning, were at the second meiotic metaphase whereas sperm nucleus retained a cupped shape of the base region. The egg chromosomes progressed to the second meiotic anaphase by 1&15 min and completed maturation by 20 min after spawning. Both female and male pronuclei were clearly differentiable at 1 hr after spawning. Female pronucleus was larger than the male pronucleus. But both pronuclei enlarged to an equal size when they were in proximity by 3 hr after spawning. Pronuclear fusion took place by 4 hr after spawning. The fused nucleus advanced into the first mitotic division rapidly so that the first mitotic metaphase or anaphase was observed by 5 hr after spawning. The first and the second nuclear divisions were completed by 6 and 8 hr after spawning, respectively. While the first and the second cytokineses occurred simultaneously at 9 hr after spawning resulting in a 4-cell embryo. Observations of chronology of the chromosomal events upon fertilization provide informations concerning the crucial times for further chromosome number manipulation.