I don't think this assumption is warranted. Human embryos develop in utero, not in eggs like amphibians. They are connected to the mother through an umbilical and immersed in amniotic fluid. Both the umbilical and the amniotic fluid are full of chemical signals in many forms, not just hormones. Human embryos (and all mammals, really) develop in symbiosis with their mother.
What you are saying is another one of these CS-centric views of the world that only focus on where the information for development comes from; that the development of the brain is somehow encoded in DNA, and that the entire development of a human is therefore also encoded in the DNA.
I've come to doubt this view, not only because of mammal development, but the fact that immune systems and digestion (the microbiome of the GI) develops in response to the available biodiversity of food and encountered pathogens.
So the view that we just put the little DNA program in a dish and feed it raw materials and energy and out pops a fully-formed human is just false.
Humans develop in cooperation with, and in response to, the environment.
It's a bit like staring at the code of a machine learning model and expecting to figure out how it works. The system (a human) contains a lot of code (DNA) but also responds to data (training set)--much more so than we ever realized.
Or like a Smalltalk programming environment, which consists of an image with live, editable objects, not just a static text program.
A neighbor comment mentions the shape of the uterus, but AFAIK this only matters in later stages of development (extrauterine pregnancies happen, and are quite deadly unless aborted since most tissues are not meant to be invaded by the placenta).
Some of the non-electric signals are given by gradients of expression of the Homeobox genes.
There is also a process of mutual differentiation where neighbouring tissues cause each other to differentiate. For example, holoprosencephaly happens when the brain fails to (fully) develop into two hemispheres.
The most severe cases results in the formation of a single eye that in turn induces gross face deformity (Cyclopia, with a nose stub above the single eye).
The mildest cases result mostly in normal people with a lack of smell and a single top central incisor.
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I don't get it. Where does that electrical guidance came from? The brain. Where did the brain came from? The DNA.
If DNA is the ony information that makes it from a parent to offspring (and we can grow organisms in a petri dish from fertilized eggs) is the DNA, then whatever electrical guidance or other abstraction must have been encoded in the DNA first. (Plus some constants encoded in the enviroment.)