. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. Figure 1. A schematic drawing showing the distribution of cyto- plasmic determinants in unfertilized and fertilized ascidian eggs. (A) In the unfertilized egg, the myoplasm (bold line) is situated in the egg periphery. The ectoplasm (white region) is located inside the germinal vesicle, and the endoplasm (stipled region) fills the inner egg region. (B) After an egg is fertilized, the myoplasm moves into the vegetal pole region. The ectoplasm forms a band of cytoplasm above the myoplasm, and the endoplasm resides above the ec
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. Figure 1. A schematic drawing showing the distribution of cyto- plasmic determinants in unfertilized and fertilized ascidian eggs. (A) In the unfertilized egg, the myoplasm (bold line) is situated in the egg periphery. The ectoplasm (white region) is located inside the germinal vesicle, and the endoplasm (stipled region) fills the inner egg region. (B) After an egg is fertilized, the myoplasm moves into the vegetal pole region. The ectoplasm forms a band of cytoplasm above the myoplasm, and the endoplasm resides above the ectoplasm. (C) During the second phase of ooplasmic segregation, the myoplasm and ectoplasm shift into the equatorial region. At the end of the second phase of ooplasmic seg- regation, the egg cytoplasm is divided into five regions: (1) ectoplasm: (2) neuroplasm; (3) notochord plasm; (4) myoplasm: and (5) endo- plasm (based on Conklin, 1905). vent subsequent cell divisions. Treatment with cytocha- lasin facilitated the identification of myoplasm-contain- ing blastomeres in eight-celled embryos after they were cultured for the time required for control embryos to de- velop into hatched larvae. When these embryos were ex- amined for the activity of acetylcholinesterase (AchE), a muscle-specific enzyme, a maximum of four blastomeres comprising a compressed embryo expressed AchE activ- ity, whereas in non-compressed eight-celled embryos a maximum of two blastomeres exhibited AchE activity. These elegant experiments performed by Whittaker pro- vide rigorous support for Conklin's idea, published in 1905. that ascidian eggs contain muscle cell determi- nants. Bissection of fertilized eggs has also been used to alter the normal distributions of myoplasm in ascidian em- bryos. When the animal hemispheres of fertilized eggs were removed, myoplasm was partitioned into more than the normal number of blastomeres (Bates. 1988). During the first phase of ooplasmic segregation, fertilized eggs w
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
