Aside from the ingredients, let’s look at the symbolism in the number of ingredients. The first witch throws in one item, the second adds nine, and the third witch adds eleven -- all odd numbers. As far back as the Ancient Greeks, odd numbers were considered to bring good luck. Since they are casting a spell to summon Macbeth in order to toy with him and bring about his ruination, the witches want to succeed. Therefore, they attend to every detail, right down to the numbers.
The total number of ingredients equals 21, also an odd number. Yet 21 has very important significance in giving power to the witches. Some believe it to have Biblical meaning as a symbol of the evil nature of sin and rebellion. For example, as the Jews wandered in the desert after being freed from Egypt, they committed 21 main rebellious acts against God. The number 21 is also said to have positive energy, as the first number, 2, can be evenly divided by the second number, 1.
As to the ingredients themselves, they are all animal or human (and very yucky) in nature, with the exception of the poisoned hemlock and “slips of yew / Slivered in the moon’s eclipse.” Being so different, the hemlock and yew beg a closer look. It is not surprising that poison hemlock symbolizes danger and death. After all, this is the very poison that the Ancient Greek philosopher Socrates was forced to drink when he was sentenced to death for (according to the Athenian council of leaders) teaching false religion. The yew is a British evergreen tree that can live for 2000 years or longer. The ancient Druids felt that it symbolized life born out of death, since it’s old branches grow to the ground, then produce new trees, which gradually merge with the original. By Shakespeare’s time, however, the yew had come to mainly symbolize death. And like many of the other ingredients in the witch’s “gruel,” yew is poisonous.
Just before Macbeth arrives, Hecate, the Greek goddess of witchcraft and magic, bewitches all the ingredients. So the items themselves are designed to bring death and destruction, while the manner of the spell, including the numbers involved, are designed to bring the witches success.
Great question. There are three main properties to the ingredients:
The first witch puts in ingredients which are hallucinogenic, creating (presumably) the apparitions which appear to Macbeth.
The second witch adds animal parts dismembered from animals, and with a particular emphasis on the disgusting (eye of newt, toe of frog).
The third witch initially adds parts of animals which are a little weirder than the second witch's (scale of dragon!) but then adds in human body parts all of which have an anti-Christian theme: the Jew,the Turk and the Tartar are all non-christians.
So, in short, the ingredients represent things which bring on hallucinations, disgusting parts of supposedly poisonous, or magical, animals, and finally, things which specifically make the witches' brew a heathen, unChristian potion.
The website below has fantastic information about all of the individual ingredients that the witches put in. I recommend it.
Hope it helps!