Full Idea
Along the guiding thread of the body. When protoplasm divides ½ + ½ does not = 1, but = 2. Thus the belief in the soul as monad becomes untenable.
Clarification
A 'monad' is Leibniz's idea of an irreducible atom of mind
Gist of Idea
With protoplasm ½+½=2, so the soul is not an indivisible monad
Source
Friedrich Nietzsche (Writings from Late Notebooks [1887], 02[68])
Book Reference
Nietzsche,Friedrich: 'Writings from the Late Notebooks', ed/tr. Bittner,Rüdiger [CUP 2003], p.71
A Reaction
This is presumably an anticipatory remark about the cutting of the corpus callosum (in the brain), which seems to cut a physical person into two people. Personally I always found the absolute unity of the mind or person implausible.