Embryonic Forces: Spacing of Feathers and Hair Follicles Reducing tensions inside embryos causes spatial irregularities and major birth defects, as extreme as doubling of organs.      These photographs show the development of the embryonic rudiments of feathers in pieces of bird skin. When the skin develops normally, on the embryo's surface (or if it is kept under mechanical tension) then the feather rudiments form with highly-regular spatial periodicity. But when skin is cultured without the normal amount of mechanical tension, then the feather rudiments form at irregular positions.     The coarse hairs on a dog's nose (vibrissae) also have very regular spacing.          Polarized light is one of the only methods by which one can "see" the complicated geometric patterns of mechanical stresses inside developing embryos.   polarized light view of squashed developing chicken wing