What is the skeleton of an embryo mainly composed of?

(! ) what’s fracking and what does it have to do with carbon, carbon dioxide, or fossil fuels?

Each pair of clay balls represents two planetesimals. if each planetesimal is composed of the same material and is separated he the same distance, which pair experiences the frayed gravitational attraction?

Which was most likely an effect on society that resulted from improvements in blood handling during world war i and world war ii?