Question 1: Embryonic chickens are a favorite organism in developmental biology research because they are very similar to mammalian embryos. Chick embryo somites form along the neural tube a couple of days after putting a fertilized egg into a warm incubator or being cared for by a hen. The somites are visible for 4 or 5 more days before their cells disperse to produce various kinds of tissues like skeletal muscle, bone, or connective tissue. If cells are taken from a somite soon after it is formed and transplanted into a different region of another somite, the transplanted cells form whatever tissue is 'supposed to be' formed by that somite region: cells taken from a muscle-forming region of one somite and put into a bone-forming region of another somite will form bone, not muscle. But, if one waits a couple of days after somite formation to take cells for transplantation, the older somite cells always form the kind of tissue specified by the region of somite from which the cells were taken: cells from a muscle region will still make muscle tissue even when transplanted into a bone-forming region. What could have happened to the later-transplanted cells that kept them from forming bone and ensured they formed only muscle?

Question 2: An 8-cell frog embryo can be separated into its individual cells and those cells placed in incubators with all the necessities for development. Each cell can divide and produce more cells but none of the cells can produce anything resembling a normal tadpole. Mouse 8-cell embryos can be separated into individual cells and those cells individually re-implanted into a female uterus. Each of the cells can produce a complete, normal embryo and ultimately a normal mouse. Why can mammals be cloned this way while amphibians cannot?