Hundreds of thousands of times a year a single-celled zygote, smaller than a grain of sand, transforms into an amazingly complex network of cells, a newborn infant. Through cellular differentiation and growth, this process is completed with precision time and time again, but very rarely a mistake in the "blueprint" of growth and development does occur. Following is a description of how the pathways of this intricate web are followed and the mistakes which happen when they are not. The impressive process of differentiation changes a single- ....