This image from the dream is in fact the first presented by Freud 40 pages earlier (pb. 448), as he notes his association ("at which I arrived with some difficulty") to stanniol, a derivative of tin, and thence to Stannius:
...the author of a paper on the nervous system of fish, which I had greatly admired in my youth. The first scientific task which my teacher [Brücke] set me was in fact concerned with the nervous system of a fish, Ammocoetes. (Freud, 1900, pb. 448).Note Freud's interpretation earlier in Chapter 6 of someone "pulling off" a large piece of tin in his dream as a symbol for masturbation (pb. 399-400).