Julia Barlow Platt studied neural crests in animal embryos and became involved in politics in the US during the nineteenth and twentieth centuries. She researched how body and head segments formed in chicks (Gallus gallus) and spiny dogfish (Squalus acanthias). Platt observed that in the mudpuppy (Necturus maculosus), the coordinated migration of neural crest cells in the embryo produced parts of the nervous system, bones, and connective tissues in the head. Platt's research indicated that the neural crest functioned like a germ layer, it challenged existing theories of what sorts of tissues arose from each of an embryo's germ layers, and it described early developmental stages of the nervous system.

Leonard Hayflick studied the processes by which cells age during the twentieth and twenty-first centuries in the United States. In 1961 at the Wistar Institute in the US, Hayflick researched a phenomenon later called the Hayflick Limit, or the claim that normal human cells can only divide forty to sixty times before they cannot divide any further. Researchers later found that the cause of the Hayflick Limit is the shortening of telomeres, or portions of DNA at the ends of chromosomes that slowly degrade as cells replicate. Hayflick used his research on normal embryonic cells to develop a vaccine for polio, and from HayflickÕs published directions, scientists developed vaccines for rubella, rabies, adenovirus, measles, chickenpox and shingles.