> The yolk is basically a bird embryo's packed lunch from mom, providing it with all the nutrients it needs to eventually grow into a baby bird ready to hatch. The arrangement is an efficient one, but it also presents two major issues for scientists trying to clone a bird. First, the yolk's size prevents it from fitting under a microscope to conduct the necessary work. And second, even if scientists could inspect the yolk on a microscopic level, finding that tiny nucleus floating somewhere in the yolk is extremely challenging. Novak has described the process as akin to "looking for a white marble in a pool of milk." > > Another issue with eggs is that once the yolk leaves the ovary, it is always on the move. During the typical cloning process with a mammal, researchers can just stick the embryo in a surrogate mother’s uterus and let it grow. Birds have no such incubation chamber. After the yolk forms, it’s dropped into something called the oviduct, where it tumbles down an assembly line that coats it with first the egg white, and then the shell membrane. There is no uterus equivalent in which to stick a bird clone embryo.