Fossils can only be formed when mud, silt or tar is in the area to preserve the fossil, If erosion does not occur and allows the fossil to form, and Oxygen and microorganisms cannot decay the bones.
Pulmonary vein
Aorta
Pulmonary artery
Vena cava
Rough endoplasmic reticulum contains ribosomes, while smooth endoplasmic reticulum does not contain ribosomes. The ribosomes are what give rough endoplasmic reticulum its characteristic "roughness."
Speed up chemical reactions (Catalysts)
2 because the 1st energy level can only hold 2 electrons and the 2nd holds 4
