Fillet of a fenny snake,... the recipe they are reciting should not be taken literally. In that era plants typically had "secret names" which constituted part of the arcane knowledge base of the herbalist and alchemist. "Eye of newt" was mustard seed, "Serpent's tongue" was the violet, many of the "fingers" were digitalis (foxglove) blossoms.
In the cauldron boil and bake;
Eye of newt and toe of frog,
Wool of bat and tongue of dog,
Adder's fork and blind-worm's sting,
Lizard's leg and howlet's wing,
For a charm of powerful trouble,
Like a hell-broth boil and bubble...
Scale of dragon, tooth of wolf,
Witch's mummy, maw and gulf
Of the ravin'd salt-sea shark,
Root of hemlock digg'd i’ the dark,
Liver of blaspheming Jew,
Gall of goat and slips of yew
Sliver'd in the moon's eclipse,
Nose of Turk and Tartar's lips,
Finger of birth-strangled babe
Ditch-deliver'd by a drab,
Make the gruel thick and slab.
Add thereto a tiger's chaudron,
For the ingredients of our cauldron...
Witches Lore has a compilation of several hundred of these, which I can't seem to transcribe here because of the columnar formatting. Those interested can browse the list at this link.