In the mouse embryo, the generation of primordial germ cells (PGCs) from the epiblast requires a bone morphogenetic protein-4 (BMP4) signal from the adjacent extraembryonic ectoderm. In this study, we report that Bmp8b, a member of the Gbb-60A class of the BMP superfamily, is expressed in the extraembryonic ectoderm in pregastrula and gastrula stage mouse embryos and is required for PGC generation. A mutation in Bmp8b on a mixed genetic background results in the absence of PGCs in 43% null mutant embryos and severe reduction in PGC number in the remainder. The heterozygotes are unaffected. On a largely C57BL/6 background, Bmp8b null mutants completely lack PGCs, and Bmp8b heterozygotes have a reduced number of PGCs. In addition, Bmp8b homozygous null embryos on both genetic backgrounds have a short allantois, and this organ is missing in some more severe mutants. Since Bmp4 heterozygote embryos have reduced numbers of PGCs, we used a genetic approach to generate double-mutant embryos to study interactions of Bmp8b and Bmp4. Embryos that are double heterozygotes for the Bmp8b and Bmp4 mutations have similar defects in PGC number as Bmp4 heterozygotes, indicating that the effects of the two BMPs are not additive. These findings suggest that BMP4 and BMP8B function as heterodimers and homodimers in PGC specification in the mouse.

School-based assessment (SBA) has recently been introduced into the Hong Kong Certificate of Education Examinations (HKCEE) in English. The present study was conducted within the context of this assessment change to investigate students' and parents' perceptions of the impact of SBA. Two surveys were employed to explore students' and parents' perceptions of SBA and their perceptions of the impact of SBA on learning. The results from the student survey demonstrated a relationship between students' perceptions of SBA-related learning activities and their perceptions of their own language competence. The results also showed significant differences between students' perceptions of the learning activities they had taken part in during the previous school year and those they were currently engaging in. In addition, parents' perceptions of SBA and the opportunities for them to know about SBA significantly and positively predicted their support for their children's SBA learning. Parents' education level and the amount of time they spent with their child daily also predicted their support for their children's SBA learning, though to a lesser extent. Further, parents' perceptions about the SBA are directly and significantly related to their children's perceptions about SBA. Taking the results from both students' and parents' surveys together, we have gained a better understanding of the complexity of the impact of SBA within the Hong Kong educational context, as perceived by students and their parents. Keywordsimpact and consequences, school-based assessment (SBA), students and parents As part of the worldwide movement to combine assessment of learning with assessment for learning in order to promote students' learning, standards-referenced school-based assessment (SBA) has recently been introduced into the Hong Kong Certificate of Language Testing 28(2) 221-249

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