Cats and dogs are born inside of the amnion that is cut opened by their mothers and eaten.
He tells Kishin that Mrs. Aiba sabotaged a Tagruato experiment involving the monster called Splinter of Amnion (known to the cult as God's vestige) and fused its DNA (in the form of a tiny ball) with Kishin's in order for the cult to use him as a vessel to control the monster, as the Splinter requires a host.