How cell develop from unicellular to multicellular?

Question

How cell develop from unicellular to multicellular?

Answer 1

A cell doesn't develop, but species did.  

The intermediate stage is colonialism.  There are some loosely aggregated examples of colonies and some that are very tightly organized like Volvox and sponges.  There are some species of sponge that you can separate the cells out by passing them through cheesecloth, and the cells will eventually reaggregate on the other side to reforme the sponge.

When the cells become more and more specialized in these colonies, eventually the cells are no longer able to live on their own and the organism is truly multicellular.