Hexagram 17 and 18

    The name of hex.17 says literally ‘follow like mashed meat’: follow without any resistance. One can follow good or bad things. Like a violin: a cheap one resonates to many tones, pure ones and impure ones. A splendid violin resonates only to pure tones. It follows vibrations without resistance because it has them in its own soul.

  The tag of hexagram 18 is a dish with worms in it. Worms are a symbol of death, but also a symbol of the beginning 'nothing' which will become a new and living creature. Snakes are a symbol of rebirth, they discard the old and worn-out skin and come out as a new creature, able to grow bigger than before.
  The sacrifices of the Shang and later Zhou were made to ‘stem-fathers’ and ‘stem-mothers’. Ancestor Geng or ancestress Gui. This is probably the meaning of the lines (see the excellent book of S. J. Marshall). When there was sickness or other bad fortune, the oracle was asked which one of the stem-ancestors was angry. Usually because his or her sacrifices had been neglected.

  Every cell of your body contains the genes you inherited from your parents, and before them from your ancestors. If you do what they prescribed, without any creativity of yourself, you will never be you. But if you discard them or even despise or hate them, you hate the contents of your own cells. You cause your cells, your body, to become weak and eventually sick. So honor your father and mother, all your ancestors, but without being their slave. Your own life has to be honorful too.
  This is the way to create a healthy and strong life and mind.

(more about hex.18)
  Jia is the first Celestial Stem, and the first day of the ten-day 'week'. It is yang wood and it indicates the beginning of things or times. Wood (or wind) is the element of growing, moving, penetrating. Before something new can grow, remove all decay, and when it has started, make sure this beginning stays clean. 
  The ideogram of jia is a shell or a buffcoat, but according to Wang HongYuan: a pole with a cross for measuring the sun and according to Li Leyi: an armor suit.
  Three days before Jia, three days after Jia .. Hexagram 15, 3 days before 18, is the moment of full moon. According to the 'Taoist I Ching' (tr. Thomas Cleary) Jia is the 'start' of the full moon. (See hex.57-58)
page 91:
   However, the benefit of crossing great rivers involves action and process, from which no deviation can be admitted. "Three days before the start, three days after the start" is the secret of the active process. The yin and yang in people is like the waning and waxing of the moon. In the first three nights of the moon's appearance, the yang light appears in the south-west. This is represented by the trigram thunder . On the night of the fifteenth, the moon appears in the east. This is represented by the trigram heaven . On the thirteenth, fourteenth, and fifteenth, the light becomes round; this is “three days before the start." On the sixteenth, seventeenth, and eighteenth, the light starts to wane; this is “three days after the start." When people's yang energy is pure, this is "three days before the start." Once their yin energy has arisen, this is “three days after the start."

Putting several sorts of poisonous insects into a closed vessel until there is only one surviving: this is Gu. Witchcraft in order to harm someone, like piercing dolls, and the evil forces which exorcists expel are also Gu. I think in this hexagram it is especially the last kind of Gu which is important, the decay or disease of the mind. Covering the whole range, from temporary lack of mental energy up to a serious mental disease, and from a moment of losing one's bearing up to a mind without ethic.

Gu poison being expelled by an exorcist from a victim. (Picture from 'The Elemental Changes' by Tang Hsiung, tr: Michael Nylan)

  Epiphanius, Ancoratus 84: 
'When the bird (Phoenix) is dead, totally burned, and also the fire extinguished, then only the raw rests of the flesh will be left over. Out of these comes into existence in one day an insignificant worm, which gets wings and renews itself. Yet on the third day it matures, and ripened due to the resources of the spot it shows itself and rises again to its own homeland and rests'.