Zells receive signals.

minor issue
in TurtleCell

receive: signal

should be

receive: aSignal

What are signals? Naming convention for parameters in Smalltalk

a / an plus class name 

Thus

aZell receive: aSignal

There is no class called Signal.

Signals are dictionaries

Example

a Dictionary(#add->#one #target->#shelly 0->#canvas )

Which conventions govern the content of the dictionary?