Brains II

Or, if you are a Roman living in 40 BC, the umbilicus. Needs flexibility as growth is going bonkers at this point. The umbilicus elongates to be the umbilical cord. Yolk portion

gets less as a substitute nutrition source takes over with veins and arterial delivery. That needs a pump – the heart [beginning as a pulsing artery] and blood to deliver the oxygen required.] Meanwhile the segmented head end is beginning to look more and more like ancient fish, which were eel-like in many ways.There were slits for water flow between the segments (making those segment slits called GILL slits and the tissues between the gill arches. Only some go so far as to make a full slit, but there is a good correspondence arch by arch. Many fish detected vibration with their jaw structure. We use the same corresponding bones as the three inner ear bones of hearing – hammer, anvil, and stapes). They have muscles that can tune what sounds get through easiest. Human speech is the center of that tuning. New mothers are tuned exactly to infant crying and hear it regardless of noise levels.

About this zippering into a tube. Proportionally massive growth is also ongoing. This needs a whole lot of DNA production as cells are dividing exponentially. Folate insufficiency is a serious risk to zippering if the mother is on a trendy diet, or staying esthetically thin without regard to what is actually in the diet.