The fish, the next transformational form of the mouse, represent the water element.
Here the chthonic quality recedes into the background, and the spiritual begins.
Mrs. Brunner has quite correctly pointed out that fish are like thoughts and premonitions that rise from the unconscious.
There is an analogy in alchemy: when the primordial water, the humidum radicale, is sufficiently heated up, something like fish eyes (that is, steam bubbles) appears in it.
This is what is most precious in fish, that which is capable of being illuminated.
We may here interpret the fish, therefore, as the transition into spiritual element, into the air.
In creation there was only the primordial water at first, which also contained the air.
Then, it says, God divided the waters which were under the firmament from the waters which were above the firmament.
The lower waters border the underworld, but the upper waters are the spirit.
According to alchemical philosophy, the spirit of life becomes visible here, the Holy Spirit.
The upper waters form the body of the pneuma and are a kind of corpus spirituale, or a spiritus corporalis, a subtle spirit.
At the final stage, humans come into being. ~Carl Jung, Children’s Dreams Seminar, Page 365.