Turns out it is genetic instructions plus cell signaling in cooperation:-http://www.livescience.com/52905-franken-flatworms-grow-heads-brains-of-other-species.html-"The research is an example of how development is controlled by more than genetics alone. The researchers did not alter the flatworms' DNA in any way, but instead manipulated proteins that control conversations between cells.-"'It is commonly thought that the sequence and structure of chromatin — the material that makes up chromosomes — determine the shape of an organism, but these results show that the function of physiological networks can override the species-specific default anatomy," study researcher Michael Levin, a biologist at Tufts University, said in a statement. -***-"First, the scientists cut off the heads of their lab specimens. Next, to alter the flatworm's regenerating head, Levin and his colleagues interrupted protein channels called gap junctions. Cells send electrical impulses through these junctions in order to communicate. The researchers found that they could easily nudge the worm to develop head and brain shapes similar to those of other closely related flatworm species.-***-"These findings raise significant questions about how genes and bioelectric networks interact to build complex body structures," Levin said. If genes provide a blueprint for an organism's body, cells are like the construction workers required to turn the plan into a structure — and gap junctions are the walkie-talkies these workers use to communicate. Disrupt the communications, and you can disrupt the building process."-Comment: Construction of body plan is even more complex than thought. Cell responses sound very automatic to me.