Complex sentence: Endocasts are formed either naturally, when a skull is filled with sediment that solidifies and fossilizes, or artificially 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 artificially or naturally, when a skull is filled with sediment that solidifies and fossilizes. Endocasts can be artificially created in the laboratory by casting the interior of the skull with a casting medium such as latex or silicone.