Complex sentence: Endocasts are formed either naturally, when a skull is filled with sediment that solidifies and fossilizes, or artificially; endocasts may also be created in the laboratory by casting the interior of the skull with a casting medium such as latex or silicone.
Split into simpler sentences: Endocasts are formed either naturally when a skull fill with sediment that solidifies and fossilizes. Endocasts may also be created in the laboratory by casting the interior of the skull with a casting medium such as latex or silicone.