Hi!
In the very nice chapter "Image Processing and Computer Vision", under paragraph "Thresholding", you're using a version of the popular "Blobs" sample image and labelling it "cells". I think the image more likely displays "gold particles", as annotated in "Computation of watersheds based on parallel graph algorithms", Meijster & Roerdink, in "Mathematical morphology and its applications to image and signal processing", Maragos P., et al. 1996.
See this thread for the figure https://twitter.com/jmutterer/status/1316510229119619072
Sincerely,
Jerome.