What does the ingredients of the cauldron signifies in Macbeth?

Probably they were chosen to sound magical ("scale of dragon"), rare ("tiger's chaudron"), poisonous (yew and hemlock) or disgusting ("finger of birth-strangled babe"). But an argument can be made that sometimes they reflect images from elsewhere in the play. Thus the "birth-strangled babe" reflects the child Lady Macbeth would have "dashed the brains out" as well as Macduff's murdered "babes". The sow who has eaten her nine farrow reflects Duncan's horses that did eat each other.

Many of the creatures are of an ambivalent nature: bats (half bird half beast), amphibians (half reptile half fish), and blindworms (half snake half lizard) which reflect the "fair is foul" theme. "Adder's fork" reminds us of looking like the flower but being the serpent under it.