Download DNA and the Developing Embryo

Adult fish, chickens, dogs, and lizards don't look
much like humans. So why do these embryos look so
much alike? The basic design of all these animals is
more similar than you might think.
Since all vertebrates (animals with backbones)
evolved from a common ancestor, the genetic
information that guides their development is nearly
the same. That's why scientists can learn about
human development by studying other organisms-including zebrafish.
DNA and the Developing Embryo
Inside almost every cell in every living organism is a long, twisted,
ladder-like molecule known as DNA. The information contained in the
DNA molecule provides a "blueprint," or a set of codes, for building
other molecules used by the cell.
As the organism grows, different parts of the DNA molecule, called
genes, are decoded and read by the cells. Each gene contains instructions
for building a particular molecule that's needed by the growing body.
As the organism develops, different genes in different cells may be read.
The ultimate fate of any one cell--whether it becomes a skin cell, a nerve
cell, a kidney cell, or a bone cell--depends upon which genes are read. If
any of the genes are missing, if they are misread, read out of sequence, or
altered in any way, the cell or organism may dramatically change.
The timing of this process depends on the embryo's stage of development
and the location of the cell. Developing eye cells, for instance, use
different combinations of genes than do brain cells or skin cells. The
embryos here are all in the same stage of development. Their physical
similarities and differences correspond to variations in their genes.
Source: http://www.exploratorium.edu/exhibits/embryo/embryoflash.html
Source: http://www.pbs.org/wgbh/nova/evolution/guess-embryo.html