Along the lines of symmetry in the future, the first manifestation is the rib cage and then the lungs. This all pervasive symmetry reaches the extreme in the outstretched arms. I think it is very interesting, and important, that again, as in the legs becoming the torso, a symmetrical duality (the arms) becomes a singularity (the neck). The similarity between two regions of the body may lead to similarity in interpretation of what the two regions mean. The contrast between the two regions is probably even more valuable on terms of interpretation.
The neck is in fact the narrowest cross-section of the body and also the purest form of singularity, since the singularity manifests itself externally and internally. This physiology suggests a very small and concentrated population at that time. It is also significant that the spinal column takes up the largest percentage of the body cross-section that it ever will. The question remains as to how the spinal column is best interpreted.