Which would be true if the DNA specification for that particular part of the body was the only part of what specifies the brain. Myers is pointing out that assertion is patently false. The environment of developing creature and the interactions between cell types and their environment (and themselves) is a giant information content multiplier, and the DNA need not explicitly specify any of this information for it to exist and be relevant.
Bringing this to a familiar compsci example; imagine software for creating neural net recognizers. You can look at the source code for a net and say, "This will take N inputs and produce N outputs" You can look at a finished classifier and say, "Ah, I see what this does! It tells the airbags in this car when to deploy!" But that's as far as you can go without the training data that was used to train the classifier. This is a doubly good example because it's often very difficult to determine HOW a complex neural net is doing what it does, but it's fairly easy to explain how to train one to do that task.