Hendry's First Law of Lamination

Hendry's First Law of Lamination: Layers exist in the central nervous system where a single structure performs more than one function. That is true because as neurons develop those of like type aggregate whereas those of different types segregate. So when you see a layer you are looking at a developmental phenomenon in which neurons of different types have gotten close to one another but have not intermixed.

In the mammalian retina, three layers of cell bodies (two of which are called nuclear layers because they contain the nuclei of those neurons) are separated by two layers of synapses (called plexiform layers). In addition, the two surfaces of the retina are formed by layers with special properties.