I’ve only just noticed that I have a fondness for food metaphors when talking about development — gastrulation is a peculiar way to make a jelly sandwich, neurulation is like rolling up a burrito, and somite formation is a meatball sub. They sort of illustrate the arrangement of the tissues involved, but of course they all have shortcomings…but then explaining how the metaphor doesn’t work can be just as informative as the metaphor itself.
For instance, early in its development, the vertebrate embryo consists of two epithelial sheets, the epiblast and hypoblast, pressed against each other like two slices of bread. That’s easy to visualize. It also allows me to explain the core idea of an epithelium — a layer of cells tightly linked to one another to form a continuous more or less two dimensional sheet. A lot of animal development is about epithelia folding and contacting other layers. But another important concept is that some cells are not in sheets — they’ve dissociated and are moving in a loose mass surrounded by an extracellular matrix. This is called mesenchyme. Mesenchyme would be the gooey jelly between the two sheet-like bread slices.